That is a great question!
When you go to bed and rise at the same time, what you are doing is keeping your circadian rhythms in balance.
If your sleep schedule is all over the map (go to bed at 1am, 9pm, 1230am, 1015p and rise at 630am, 9am, 4am, 715am), your circadian system falls out of what is known as "entrainment" with the light-dark cycles of the earth/sun. The circadian system is what all living things use to stay tuned to these cycles.
When we work overnight shifts, stay up too late, or get up too early, or take long naps during the day, we confuse the circadian system and it makes it more difficult for the brain to know when it's actually time to sleep and actually time to get up.
This is why many people are suffering from accrued "sleep debt." They do not adhere to a sleep/wake schedule that is consistent, so their systems fall out of balance. This can lead to problems with falling asleep at night, daytime sleepiness, cognitive "fog," days and productivity lost to naps, and waking up too early or too late for practical purposes.
I hope that helps!