Number one (by far) is control. Life often feels out of control and chaotic. Managing everything feels like trying to corral a bunch of pinging electrons into order (I guess I don't actually know what that is like, but you know what I mean). A list gives at least an illusion of order - of a finite set of tasks that has an end. And of course there is that satisfaction of crossing things off the list. They can't all be long term list items or there would never be any sense of progress, thus the practice of including "buy coffee" or "call for oil refill" alongside "organize everyone's summer calendars" and "work on chapter 3."
There are always those things that stay on the list for months, and sometimes eventually disappear out of apathy, and other things that reappear perpetually - laundry! mail, bills. But generally, a list is a highly useful tool for defining an amorphous life and making it feel more manageable.
The second thing about lists seems kind of paradoxical next to the first. In spite of the fact that life is so packed a lot of the time, there are increasing times nowadays when life feels frighteningly empty. There are always long term projects one could dive into, but there is no real urgency or deadline unless it is self-imposed. There are times, especially coming off of a crazily busy stretch, when suddenly it feels as though there is nothing important to do. That feeling can be as bad as having too much.
In this case, lists come to the rescue again. Aha! There are actually a lot of things that need doing. It is the absence of an immediate, pressing need that gives the illusion of emptiness. Life is far from empty -- make a list! It goes on a lot longer than you expected. Now pick something - start somewhere. You now have a sense of direction, and surely more things are going to come along demanding your time.
And it's okay to include "finish reading that book" on a list too. A list can also be a place to give yourself permission to sit down.