I have two pool lots in my game, one is a upscale indoor/outdoor public swimming pool with adult entertainment facilities as well, bar, dj booth, dance floor. The other is an outdoorsy natural looking pool with some other outdoorsy things to do, like picnic tables, campfires, etc. that I set as a teen hangout.
I think the main purpose of it depends on how you approach gameplay. I like to have a world for my sims to experience, and in the world there are places to swim that are not gyms and reasons to go swimming that aren't related to getting fit.
The main "pro" I would say is that AI behaviour of sims that you do not control is determined by the lot type, so on a gym lot, pools won't get used much by the AI, whereas on a pool lot they do. Additionally, what non-active sims are generated to come to the lot is determined by lot type, pools will get children as well as adults, for example.
If you approach the game more in terms of "i need objects x, y and z for my sims to gain skills a, b and c", and couldn't care less about what other non-active sims come and what they do, then a pool lot type doesn't really serve a purpose.