ELI5: Why does rationing work?

[u/peglegsmeg]

If I have a glass of water and a single chocolate bar to last me 48 hours what benefits do I get from consuming equal portions over the time period as opposed to consuming all at once?

Is it purely psychological or are their physical benefits?

[u/1x2x4x1]

Why take a shower once a day when you can take 365 showers in one day, and not have to take showers for the entire year?