Star Fortresses allows you to get more companions and to do some achievement hunting.
To do the heroic Star Fortresses, you need a toon that as done the chapter 9 of Knight of the Fallen Empire.
To start the quest line, you need talk to Miot Dengd in your base on Odessen. You need to do one story mode and than you have access to the 6 Star Fortress alliance alert for the heroic ones.
It would be preferable to have done the first story mode star fortress(they are quite easy, you shouldn't have any issue doing them solo) and the bunkers (first part of the alerts) for the heroics. That way we can do more Star Fortress heroic mode and maximize our time.