Allaah, the Exalted said,
"Say to the believing men that they should lower their gaze and
guard their private parts; that will make for greater purity for them. Indeed Allaah is well acquainted with all that they
do." [an-Nur (24):30]
So Allaah made purification and spiritual growth to be the outcome of lowering
the gaze and guarding the private parts. It is for this reason that lowrering ones gaze from (seeing) the prohibited things
necessarily leads to three benefits that carry tremendous value and are of great significance.
The First: experiencing the delight and sweetness of
This delight and sweetness is far greater and more desirable that which might
have been attained from the object that one lowered his gaze from for the sake of Allaah. Indeed, "whosoever leaves something
for the sake of Allaah then Allaah, the Mighty and Magnificent, will replace it with something better than it." 
The soul is a temptress and loves to look at beautiful forms and the eye is
the guide of the heart. The heart commissions its guide to go and look to see what is there and when the eye informs it of
a beautiful image it shudders out of love and desire for it. Frequently such inter-relations tire and wear down both the heart
and the eye as is said:
When you sent your eye as a guide For your heart one day, the object of sight
fatigued you For you saw one over whom you had no power Neither a portion or in totality, instead you had to be patient.
Therefore when the sight is prevented from looking and investigating the heart
finds relief from having to go through the arduous task of (vainly) seeking and desiring.
Whosoever lets his sight roam free will find that he is in a perpetual state
of loss and anguish for sight gives birth to love (mahabbah) the starting point of which is the heart being devoted and dependant
upon that which it beholds. This then intensifies to become fervent longing (sabaabah) whereby the heart becomes totally dependant
and devoted to the (object of its desire). Then this further intensifies and becomes infatuation (gharaamah) which clings
to the heart like the one seeking repayment of a debt clings firmly to the one who has to pay the debt. Then this intensifies
and becomes passionate love (ishk) and this is a love that transgresses all bounds. Then this further intensifies and becomes
crazed passion (shaghafa) and this a love that encompasses every tiny part of the heart. Then this intensifies and becomes
worshipful love (tatayyuma). Tatayyum means worship and it is said: tayyama Allaah i.e. he worshipped Allaah.
Hence the heart begins to worship that which is not correct for it to worship
and the reason behind all of this was an illegal glance. The heart is now bound in chains whereas before it used to be the
master, it is now imprisoned whereas before it was free. It has been oppressed by the eye and it complains to it upon which
the eye replies: I am your guide and messenger and it was you who sent me in the first place!
All that has been mentioned applies to the heart that has relinquished the
love of Allaah and being sincere to Him for indeed the heart must have an object of love that it devotes itself to. Therefore
when the heart does not love Allaah Alone and does not take Him as its God then it must worship something else.
Allaah said concerning Yusuf as-Siddeeq 'alayhis salaam,
"Thus (did We order) so that We might turn away from him all
evil and indecent actions for he was one of Our sincere servants." [Yusuf (12): 24]
It was because the wife of al-Azeez was a polytheist that (the passionate love)
entered her heart despite her being married. It was because Yusuf 'alayhis salaam was sincere to Allaah that he was saved
from it despite his being a young man, unamarried and a servant.
The Second: the illumination of the heart, clear perception
and penetrating insight.
Ibn Shujaa` al-Kirmaanee said, "whosoever builds his outward form upon following
the Sunnah, his internal form upon perpetual contemplation and awareness of Allaah, he restrains his soul from following desires,
he lowers his gaze from the forbidden things and he always eats the lawful things then his perception and insight shall never
Allaah mentioned the people of Lut and what they were afflicted with and then
He went on to say,
"Indeed in this are signs for the Mutawassimeen." [al-Hijr
The Mutwassimeen are those who have clear perception and penetrating insight,
those who are secure from looking at the unlawful and performing indecent acts.
Allaah said after mentioning the verse concerning lowering the gaze,
"Allaah is the Light of the heavens and the earth."
[an-Nur (24): 35]
The reason behind this is that the reward is of the same type as the action.
So whosoever lowers his gaze from the unlawful for the sake of Allaah, the Mighty and Magnificent, He will replace it with
something better than it of the same type. So just as the servant restrained the light of his eye from falling upon the unlawful,
Allaah blesses the light of his sight and heart thereby making him perceive what he would not have seen and understood had
he not lowered his gaze.
This is a matter that the person can physically sense in himself for the heart
is like a mirror and the base desires are like rust upon it. When the mirror is polished and cleaned of the rust then it will
reflect the realities (haqaa`iq) as they actually are. However if it remains rusty then it will not reflect properly and therefore
its knowledge and speech will arise from conjecture and doubt.
The Third: the heart becoming strong, firm and courageous.
Allaah will give it the might of aid for its strength just as He gave it the
might of clear proofs for its light. Hence the heart shall combine both of these factors and as a result, Shaytaan shall flee
from it. It is mentioned in the narration, "whosoever opposes his base desires, the Shaytaan shall flee in terror from
his shade." 
This is why the one who follows his base desires shall find in himself the
ignominy of the soul, its being weak, feeble and contemptible. Indeed Allaah places nobilty for the one who obeys Him and
disgrace for the one who disobeys Him,
"So do not lose heart nor fall into despair; for you must gain
mastery if you are true in faith." [Aali Imraan(3): 139]
"If any do seek for nobilty and power then to Allaah belongs
all nobility and power." [Faatir(35): 10]
Meaning that whosoever seeks after disobedience and sin then Allaah, the Might
and Magnificent, will humiliate the one who disobeys Him.
Some of the salaf said, "the people seek nobilty and power at the door of the
Kings and they will not find it except through the obedience of Allaah."
This is because the one who who obeys Allaah has taken Allaah as his
friend and protector and Allaah will never humiliate the one who takes his Lord as friend and patron. In the Du`aa Qunut their
occurs, "the one who You take as a friend is not humiliated and the one who You take as an enemy is not ennobled."
Reported by Ahmad [5/363], al-Marwazee in 'Zawaa`id az-Zuhd'
[no. 412], an-Nasaa`ee in 'al-Kubraa' as mentioned in 'Tuhfah al-Ashraaf' [11/199] from one of the Companions that the Messenger
of Allaah (SAW) said, "indeed you will not leave anything for the sake of Allaah except that Allaah will replace it with something
better than it." The isnaad is saheeh.
This is not established as a hadeeth of the Prophet (SAW)
Reported by Abu Daawood [Eng. Trans. 1/374 no. 1420], an-Nasaa`ee
[3/248], at-Tirmidhee [no. 464], ibn Maajah [no. 1178], ad-Daarimee [1/311], Ahmad [1/199], ibn Khuzaymah [2/151] from al-Hasan
from Alee (RA). The hadeeth is saheeh. The isnaad has been critcised by many, however none of the critcisms hold. Refer to:
'Nasb ar-Raayah' [2/125] and 'Talkhees al-Habeer' [1/247]