In a game where FTL travel is constrained by stellar geometry, a stellar stream coming into navigability with the players’ chunk of the galaxy could become a source of many new plotlines. The stream could bring in significantly different life forms, possibly completely new to the galaxy, or the result of millions of years of divergent evolution. Merchants would be keen to open up new markets, prospectors would be eager to exploit anything new and different in these uncharted territories, and colonists with a planet fetish who just aren’t happy living in orbital megastructures might jump at the chance to find some new soil under their feet. They’re a fine way to shake up a campaign setting.
Streams could also be significant for military strategy. The Sagittarius stream serves as a bridge from the Milky Way to the orbiting Sagittarius dwarf elliptical galaxy. The stream alone contains 100 million stars and stretches a million light-years, wrapped around the Milky Way. Any stellar empire bent on conquering a new galactic arrival would need to go through that bottleneck. (Using a dwarf galaxy as a vehicle for intergalactic conquest would require multi-billion-year patience, though. Of course, if you could put Shkadov thrusters on enough stars in a dwarf galaxy, you could take the whole thing cruising...)