Everything I'm planning to do that day goes on the list. Even if it's low-priority or non-urgent. Even obvious stuff that's part of my routine like "take a shower" or "eat lunch". And leisure stuff like "watch football game". If I really intend to do it today, it gets included.
One benefit is it helps me avoid being over-optimistic about how much time I have. If only certain select items are written down, it's too easy to look at it and get a false idea that I don't have that much to do.
The other benefit is I find it easier to think about. If some items are implicitly going to happen and others are written down, then working with the list is a process of trying to simultaneously read some things and remember others. It just works better if I see everything all together in one place.