For me, it is because i have to force myself to sleep on a schedule that isn't what my body wants to be on.

I'm a night owl. If i let my body set its own sleep sleep, I'd be going to sleep sometime between 6am-9am. If i ever get an extended period of time off from work and just sleep when I'm tired, instead of going to bed at a time that i can get a good night sleep in order to get up for work, that's the schedule I'll be on within a week.

I unfortunately have to leave for work at 5 am, so i simply can not let my body set its own sleep schedule. I have to go to bed at the same time 7 days a week. Just staying awake 2-3 extra hours, say on a Friday or Saturday night, means that when i need to go to bed in order to get up for work Monday morning, I'm simply not tired enough to sleep. And it'll take me 2-3 days to get back onto my forced sleep schedule, during which I'll be a zombie until I'm able actually get a good night's sleep in the schedule I'm forced to sleep.