When you sent your eye as a guide…

Allah, 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 Allah is well acquainted with all that they do.” [Surah Al-Nur 24:30]

So Allah made purification and spiritual growth to be the outcome of lowering the gaze and guarding the private parts.

As  Ibn Qayyim al Jawziyyah 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.