When, beyond theoretical exercises, do we care how many solutions we can find for something?
I had an analogous question for TMs before - why is it useful to study machines that can only deliver acceptance and rejection as output? Here, the answer (at least in a practical context) seems more clear - find a solution (e.g., a shortest path in a shortest path problem) is a matter of poly-time reduction to yes/no answers. For #P problems, reduction is equally obvious, but not useful, since you're solving a problem at least as hard.