As said above, for time-saving and convenience because they already did the legwork to make it fit for deployment, or customize for specific applications. You can still tweak it, of course, but at first it's a turn-key solution.I don't get why to use "based on Arch" if you can directly use Arch (that what I do).
I really like Arch, too, though will probably stick with the previous idea because of the developer's attention to hardware integration which is highly relevant for gaming but not many distros care about. I had pop_os once before and wasn't immediately in love with its looks, but it worked where others failed with something I wanted to try and this is what counts. Certainly development has come a long way since then, but I lost interest in chasing all these details. Here is a comparison on Slant: