What's a Space Opera without some aliens?* I'm firmly of the opinion that once you've breached the idea of humans going into space, traveling faster than light, aliens are an inevitable, logical step. Of course, how those aliens are presented, and what they do can vary wildly.
This ties to stellar geography, of course. An alien species might be a peaceful, enlightened civilization that only wants to make the prettiest baskets in the history of the cosmos, but if they have a highly advanced conqueror species in their interstellar backyard, that's the thing that's going to define them.
Now, there's always an urge to show a vast interstellar community. Regardless of other politics, there is an urge to showcase places that act as a crossroads for dozens of alien species. These places are usually bars. Of course, there is the classic Cantina Scene in Star Wars, and Star Trek, Babylon 5 and especially Farscape have all milked this trope for everything its worth. And it's fun, and it looks impressive-- though usually that's because the make-up and art design people are having a blast, and not because much thought is put into who any of these people are. The idea that they could all share a space-- comfortably in the same atmosphere and gravity-- let a lone that a single establishment could easily serve all their needs... that's challenging to believe. And more to the point, they tend to be just set dressing. B5 tended to purge the more set-dressing aliens early on, so there were very few random-aliens-of-unnamed-species. But most of the time, aliens are just thrown out there, with little thought about how or why they are there.
Me, I can't work like that. Just like I need to know the stellar geography, that includes the political geography. Neighbors matter. Tech levels of neighbors matter. If you have an area where there are several species that are FTL-capable, then any species that is not is dependent on the good graces of those that are.
In my setting, there are 11 intelligent species whose homeworlds are within 30 light-years of Earth. Of those, 5 are of starfaring technology, and three of them were starfaring before humans. Fortunately for humanity, those three were in alliance with each other, and agreed upon a rule of non-interference with planet-bound species. We thrived and reached out because they let us. Some fifty light-years away is the Surani homeworld, and they didn't share that philosophy: the two closest species to them (the Xaedon and the Dalians) were incorporated into their empire, conquered and integrated as servant classes effortlessly. And within 30 light-years of the highly advanced Rilixa, there are no intelligent species. Not one remains in what they've defined as their space.
I've done a lot of work along these lines: within 100ly of Earth, I've defined (roughly, mind you, roughly) 153 alien species, of which 69 are starfaring. For me, this was groundwork. This was just getting the lay of the land so I could get a sense of the stories I could write.
Because for me, I have to know where I am, and who's around me, before I really have a sense of what's going on.
*- Firefly or Battlestar Galactica, I guess.