I'm not sure the exact reason for this rule.“He eats only in one part of the day, refraining from food at night and from eating at improper times." - DN 2
It could be to promote health or promote self-discipline, or both.
It says in MN 21:
Seems to imply that it's for improving health, strength, comfort.“Once, I addressed them: ‘I eat my food in one sitting per day. Doing so, I find that I’m healthy and well, nimble, strong, and living comfortably. You too should eat your food in one sitting per day. Doing so, you’ll find that you’re healthy and well, nimble, strong, and living comfortably.’ I didn’t have to keep on instructing those mendicants; I just had to prompt their mindfulness." - MN 21
Then it lists as one of the five factors of meditation (for quickly achieving arahantship):
It also lists one of the five unfavorable occasions for striving:(2) “He is seldom ill or afflicted, possessing an even digestion that is neither too cool nor too hot but moderate and suitable for striving." - AN 10.11
So developing good health is an essential aspect making achieving arahantship here and now easier.(2) “Again, a bhikkhu is ill, overcome by illness. This is the second unfavorable occasion for striving." - AN 5.54