Really, the only real physiological reason I can think of is to restore glycogen levels if doing a keto-style diet. People mention these thyroid/leptin/ghrelin theories, but I've talked with a handful of people that have tried it both ways and they maintain that they got lean quicker with no cheat meals.
Galeniko & Sevastase are two posters here that I know have mentioned this
I never cheat,notice I said clean bigger meals............I know you weren`t referring specifically to me,just saying.
I`ve had clients just starting a diet and they were fat, ask me when their cheat day was.

The only time I cheat,is when I hit 4-5 % bf,and then it`s beef,more whole eggs,a few higher carb days.........not much of a cheat,just more fats/cals.
Once I am in clean eating mode,it is not a problem for me to continue eating clean,but I only do it that strict for a contest prep,other than that,I live a little and have a cheat day and a cheat meal here and there.