e.g. you might take some pills right after you wake up and some pills right before you go to bed.
If you wake up every day at 7am and go to sleep at 11pm then that's easy, but if your schedule is constantly fluctuating (go to bed at 8am and waking up at 3pm) I can imagine how it might become confusing.
With a more erratic schedule, "take this pill at 10am on Tuesday" is a more valuable reminder than with a regular schedule where you might take it triggered by a regular event.
For example if a medication has sedating effects (such as some antidepressants) it may be advised to be taken before bed so this side effect is turned into something generally beneficial (more sleep) and less prevalent during waking hours).
Statins (cholesterol lowering medications) are usually prescribed at night time as this is when your cholesterol catabolism is most active.
Pain relief is often varied around sleep schedules
Insulin is scheduled around meals and sleep
Etc etc.
Antidepressants may be advisable to take before bed as a rule, but it's an excellent example of drugs that are supposed to be taken at the same time every day. Irregularly moving around the hour you take the antidepressant makes them less likely to work.
That's why irregular sleep schedules aren't a good reason to move medication.