Here's what I remember from reading the excellent Why We Sleep by Matthew Walker, recommended elsewhere in this thread:

There are two unrelated cycles that help us get to sleep. One is our circadian cycle, or the "is it time to be awake or asleep right now?" cycle. Your body has a sense of whether it's daytime or nighttime, based on exposure to light, internal timekeeping, and some other factors. So you're naturally more awake during the day and get sleepy at night. This can get messed up when we travel to a different time zone -- we call this jet lag.

The other is the sleep pressure cycle, or the "how tired am I?" cycle. There's a chemical in our bodies called adenosine that builds up while we're awake, and gets cleared out while we're asleep. This chemical makes us feel sleepy, generating "sleep pressure" that helps us fall asleep.

So if you wake up and go to sleep at the same time every day, your sleep pressure cycle and your circadian cycle line up: when night falls, you have a lot of adenosine built up, so you can fall asleep more easily. But if you oversleep one day, you start making adenosine later in the day, so you have less at nighttime. Your circadian rhythm tells you you're sleepy, but you don't have as much sleep pressure, so it's harder to fall asleep. So maybe you lie awake for a long time and wake up late the next day, too, perpetuating the problem. For a lot of people, once the cycles get out of whack, it's hard to get them back into sync.

As other people have said, everyone's different. Some people have no problem with flexible sleep schedules. For me, making sure I go to bed and wake up at the same time every day has been AMAZINGLY helpful. I used to have terrible insomnia, and now it's much milder.

If this is something you struggle with, I definitely recommend reading Why We Sleep. It's not a self-help book, but I found it really helpful in understanding my own sleep issues.