They curate the subs that appear.
All is everything minus a deny list (nsfw).
Popular is a list of what they approve, or more recently it’s the algorithmic feed.
I only used my own subs for my personal interests or All to discover something new. It was, at least at one time, good for surfacing things I never would have looked for. More often things that didn’t interest me, but that was valuable in itself.
Or dogs with bees in their mouth and when they bark, they shoot bees at you?