As you fight your way across Volistad you link up with various forces that went to ground following the invasion. Regaining the Space Port is one of your earlier missions and the Space Port serves as a general arrival area for the Space Marines though unfortunately you only gain access to Scouts through the game.
As you progress various units join up with you and add to your existing armies or form armies on their own so you can have several fronts operational at one time. My personal favorite though is when you liberate an Adeptus Mechanicus Titan Base. Now as the background fiction for 40k has advanced since this game came out one can assume that the producers of the game just didn't get what a Tech Priest should look like as the human in the robes has no visible modifications or mechanical bits. However, in the novel Titanicus, we are introduced to characters who are part of the Mechanicus but must serve as go betweens for the Mechanicum and the Imperials. Due to this, they do have mechanical upgrades internally however they are kept as human looking as possible on the outside so as not to unsettle the Imperials they work with. I know I'm digging but linking these two items up at least excuses how human this guy looks.