Basically they'd be like planets, but in outer space.
I imagine they'd be built in a modular way with there being a special way to start one and after that it could be expanded with installations.
Possible installations:
Habitation Sphere - Provides everything needed to host a population. (Like orbital habitats)
Warehouse - This would be an actual installation, since in space you don't have the liberty of just putting resources on the ground somewhere. It would ALSO provide the space for other installations. Warehouses that have anything in them either can't be removed or whatever they were holding is lost.
Stuff they could do:
Construct installations.
Research stuff.
Host shipyards.
Have populations.
Be moved by tugs, but at a very large cost.
Stuff they could not do:
Mine, because they wouldn't be at a planet.
Why do I want this? - It would provide a somewhat viable alternative to planets, since you can move it (although slowly). They wouldn't be better than planets in every case because they can't mine anything on their own - you'd have to use asteroid miners or Automated Mines which both have their own downsides.
Alternatively, I would just like a way for orbital habitats and some installations to work in outer space. The purpose is the same - to have a borg-cube-ish flying homeworld.