Why do Buddhist monks fast ?

Buddha said to the monks to refrain from the night meal. Here are the reasons why:

"Monks: I abstain from the night-time meal.
As I am abstaining from the night-time meal I sense"-

1. next-to-no illness
2. next-to-no affliction
3. lightness
4. strength
5. a comfortable abiding

"Come now. You too abstain from the night-time meal. As you are abstaining from the night-time meal, you, too, wil: sense"-

1. next-to-no illness
2. next-to-no affliction
3. lightness
4. strength
5. a comfortable abiding

-Kitagiri Sutta-At Kitagiri

http://www.accesstoinsight.org/tipitaka/mn/mn.070.than.html