Yeah, the reasons seem to be any or all of the following:
*adding because a given med doesn't take care of all problematic symptoms
*to counteract side effects
*a belief in synergistic effect
*being afraid to take something out of the mix.
And in some cases, possibly just laziness.

I'm lucky that my provider is of the mindset of working with as few as needed to accomplish the task. (And if a dose has to be increased for a time, it's not just left there for no reason.)

I've got 4, one being PRN. No real duplicate purposes. Dosages adjusted as appropriate to situation.

Edited to add... She is a PMHNP (psych nurse practitioner), doing both my meds and therapy. I think it's a good combo because I'm not just some 5-15 minute face. She knows directly what's going on with me.