That is, at the very least, something that "regular" pirates can't do but at least air pirates can. Ocean piracy doesn't have free rein of a whole world--just the parts reachable by a relatively deep-draft cargo vessels. A sky pirate can fly bombing runs in the dead of night, or dogfight with dragons, or try to lose an enemy by flying between cloud layers.Why Spelljammer? Because it's weird. My players are absolutely not interested in playing a D&D pirate game, and aren't all that interested in a seafaring exploration game, but they're absolutely interested in playing an exploration game in weird fantasy space. (Heck come to think of it they're more interested in playing an exploration game with flying ships on the surface of the gameworld than they are a seafaring one as well - flying ships are a big draw I think).
Space adds in...more of a difference of degree than kind, but a difference nonetheless. Asteroid-hopping might be kinda-sorta analogous to hopping between floating islands, but it's not quite the same and carries a much higher risk of danger overall. If space is a vacuum, that's obviously a pretty big danger (much, MUCH more dangerous than the open sea--the open sea can take weeks to kill you, hard vacuum will do it in a matter of minutes). It's always possible to have very dangerous islands or locations, but it's hard for me to come up with something that would have quite the same tone as the classic "voyage to the sun/through the corona" kind of thing that is a very classic space fantasy but hard to finagle into even sky pirates, let alone regular boating-on-the-ocean pirates.
So, I guess that's kind of it. There are a few (not many, but a few) adventure ideas that either need space pirates, or just work a lot better/easier/more smoothly with space pirates. There are a few further ones that work more or less equally well with sky pirates as they do with space pirates, but which definitely wouldn't be easy (or even plausible) to put into a regular boating-on-the-ocean pirate story.