Basically you can pretty much consider that the booleans are kind of a shortcut wrapper that combine Trimming, then automatic detection of which pieces to discard based on where the pieces are within different volumes, then combined by Joining together the resulting pieces.

When you're working with solids, this combination action of the booleans tends to be very convenient and saves several manual type editing type steps.

But if you are working with open surfaces instead of solids, it tends to be more likely that you will use the "low level" Trim and Join type tools instead.

You can actually use surfaces in booleans in different cases as well, but when there are no volumes involved the "automatic detection of which pieces to discard" part doesn't really apply. That's when Trim becomes better because you specify exactly which pieces you want to keep and discard in that case.

- Michael