Gonna be honest, I consider myself pretty into TTRPGs and even I don't know what the deal is with Pathfinder. I looked at it when the first edition came out and it seemed like "D&D 3.75" so I didn't see a reason to bother with it when I already had so many books for 3.x. I have heard the current edition is doing its own thing but haven't bought the book to see what that thing is.
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As far as I know, Pathfinder 1E was a continuation of D&D 3.5 after they moved on to D&D 4E. I think Pathfinder 1E is compatible with D&D 3.5, but don't quote me on that. Pathfinder 2E is a much more tightly balanced system, and is probably best for its 4 degrees of success, where 10 over a DC is a critical success and 10 under is a critical failure.
Also, Pathfinder has way more options to customize your character as you level up compared to D&D 5E.
That's the whole point. Pathfinder's origin is a collaboration of people looking at DnD4 and thinking "we can do better"... And for quite some time they were right as DnD's 4th edition was really that unpopular.
Honestly, as someone who was all for a new contender in the rpg crawl/slog category, PF 1.0 certainly seemed like D&D 3.75. (and it didn't hurt that presumption that a number of OGs had left D&D to help launch it.)
Apparently, many felt that it was better at D&D than even D&D at the time, though I've since become an indifferent fan of it at best.
Same slog, same crunch, same vast array of "options" without much in the way of lowering the myriad barriers to entry, etc. Meh.
Good thing now there's Daggerheart, Blades in the Dark, Powered by the Apocalypse, Worlds/Stars Without Number, et al! 🤩🤘🏼
I mean, that was the intention. It was designed specifically for the 3.5 people to have more 3.5 to play after WotC decided to move on to 4th edition. That was the entire goal. There was also a big OGL scandal because WotC was trying to make an proprietary in-house VTT with 4e's rules coded into it, and as such they needed the rules to be legally Theirs. Paizo was in a place to take advantage of that misstep, and so they did.