Because it's just like with reading books. At first it's very slow and tedious (for some people) process. But if you stick with it, after you read many books, it's getting faster, easier, and more fun to read every new book. The same is with sight-reading.
You start recognizing patterns, chords, modulations, tonal plan, form of the piece much faster than before because you are familiar with many similar compositions already.
Besides, your manual and pedal technique is getting better by the day because you constantly encounter new patterns.
But perhaps the biggest visible benefit of all is this - you are hungry for more beautiful new music.
And because of this hunger you are anxious to learn about music theory and harmony, too. You are not satisfied with being able to play the piece, you want to fully understand what you play.
If you are hungrier still, you switch the mode - from the performer mode, you switch to the creator mode - you start creating your music based on the models you play.
Reading changes people.
[HT to Mateusz]
Organ Improvisation in Eb
Ending in a few hours: Fingering and Pedaling for Your Organ Piece
Ending tomorrow: Vidas Private Coaching