Other reasons for being on Earth

Other than being the last survivor of a dead world, I mean.  One that has occurred to me is being essentially a combination of a Green Lantern, a park ranger and border patrol; that is, law enforcement sent to protect humanity from interstellar criminals that aren’t supposed to be here and enforce an equivalent of the Prime Directive.  Another is being a scientist here for a long-term study of some kind.

In AU lore, the Earth is ManHome, the planet where all the humans who now fill the galaxy first evolved. Except nobody on Earth knows that. 

The result is that Earth is viewed by all star-faring planets as a huge, primitive, untouched natural park where humans can live naturally, free to be what they might naturally become, completely unaware that their descendants have spread to a thousand worlds and have starships and stuff. Visiting Earth is strictly limited and controlled to avoid startling the Earthlings or messing up Earth's cultures, and only then by blending in with them. 

Velorian Protectors are a bit like Rangers from Tolkien lore who kept the Hobbits safe in the Shire without the Hobbits even knowing they were being protected. They don't intervene in purely Earth issues, but keep Earth free of evil and dangerous ET's. 

I thought it would be fun to show that we Earthlings are the ONLY ones who DON'T know that humans now populate the entire Galaxy, thanks to ancient alien abduction and the Seeders, who spread humanity far and wide, seeding a thousand worlds with the most adaptable and creative species ever discovered. Some of whom have been enhanced or evolved in different directions than on Earth. 

Sort of like preserving your landrace seed stock in case the hybridization process hits a dead end somewhere else.  

But for those few Earthlings who a Protector seeks out to help them when on Earth, it's a really big deal, given they have more or less Kryptonian-grade strength and powers and usually look a bit like this:

 

Shadar

 

I mean, the obvious answer (for at least some alien races) is that Earth is a primo vacation spot because they have super powers here. If I was a Kryptonian, a pleasant world with a yellow sun would be high on my list of vacation spots.

Obviously for aliens who have super powers everywhere, that rationale would not apply.

Here an idea about the seeders and limited access to Earth for star based humans.

Maybe the seeders put something in the Earth based humans to protect Earth. Say if one of the star based humans had a kid with an Earther, the kid would make P1s look like a wimp. On top of that the kid would be have genetic memory to protect the Earth.

Maybe they're just explorers whose interstellar travel is slow enough that it takes a long time to get places. They just happen to be visiting our part of the galaxy to see if there's anything interesting to see here. They discover a populated planet and decide to stick around for a bit to see what it's like.

Or maybe they're traveling the galaxy, and their space ship broke down. So they pulled over at the nearest habitable planet for repairs, only to discover that Earth's technology isn't advanced enough to help them get home. At least, not easily.

Or you could blatantly rip off some other well known works of fiction. There's Maxima, who came to Earth looking for a worthy mate. There's Ford Prefect, who is writing an entry about Earth for an interstellar tourist guide book. There's even Superman's father, Jor-El, who in a flashback episode of the Smallville TV show came to Earth just to visit and learn about our primitive culture as a sort of "coming of age" ritual.

Taking an idea from a much lesser known past TV show, you could have an interstellar criminal who accidentally ends up on Earth while evading the authorities, and then the cops come looking. This could set up two opposing aliens (or groups of aliens) visiting Earth. In the 80s TV show Something Is Out There, the alien criminal was very evil and going around killing people, and the cop chasing it was a beautiful alien, played by "Bond Girl" Maryam d'Abo. But you could play around with the premise. Maybe the criminal is actually the good guy, on the run from an evil empire, and the authorities chasing them are the bad guys. Or maybe neither of them cares at all about Earth or Earthlings, and will leave a trail of dead humans as collateral damage in their fight against each other.