One of the tropes of many space opera settings (especially Traveller) is the scattering of primitive humans across the galaxy by an advanced alien race. Over the thousands of years in unusual environments some of these races may evolve strange abilities, morphologies and cultures. It worked for The Legion of Super Heroes. Battlestar Galactica did away with the alien travel agency entirely and simply assumed the humans somehow lost most of their technology after reaching Earth or wherever.
The benefits to using this trope is that you don't have to create an alien race from scratch (which is a lot of work) and you don't have to worry that your new race isn't weird enough.
After all they are human.
The problems with this approach is that you seldom get any clue why some advanced culture would take a bunch of cavemen (or Romans etc.) and plop them down on another planet let alone a whole bunch of planets. Some reasons:
1) Labor force- Humans are adaptable and show high initiative. On the other hand robots don't need life support, quarters, and toys. Also do you really want to teach a bunch of knuckle draggers how to operate your mining lasers?
2) Mercenaries- Some people feel humans have a talent for warfare. A race of advanced sissies might want us to fight their wars for them. This has the same drawbacks as reason 1 and giving the knuckle draggers actual weapons is even more dangerous.
3) Spare parts- This assumes the aliens seeded Earth with life and guided a lot of our evolution. They can use our blood, organs and such and want us for medical supplies. Or maybe it's merely one branch of humanity that developed spaceflight after being dumped somewhere. The drawbacks are less obvious but we're moving away from transplants now by developing stem cell therapy and we may be cloning transplant materials soon.
4) Artisans- Humans possess a skill or perspective the aliens liked. Maybe the galaxy is seeded with troupes of ballet dancers or impressionist painters or sculptors. You don't need to teach artists or entertainers your technology. When the alien patrons die off or leave the humans are left to develop technology all alone. Unless the aliens have them sculpting with mining lasers.
5) Pets/servants- Maybe having a human around the house was a status symbol (more points for a redhead which is why they're so rare here on Earth.) Maybe humans were a form of currency. They spread them thin to keep breeding populations low and avoid inflation.
6) Conservation- Humanity was nearly wiped out by geological disasters at least once. Maybe some kind aliens will transplant some of our brothers to other worlds to insure our survival.
7) It's an experiment- The aliens never went away. They are keeping a low profile and observing humanity in all its forms.
Comments are welcome.