The Trashmaster is, as the name might suggest, the garbage truck vehicle in Grand Theft Auto 5. Maybe best spawned if you have a body to dispose of... or maybe you need the "tank" like armor features and heaviness of the Trashmaster for a specific job.
Either way, the Trashmaster is pretty slow and difficult to manoeuvre, so if you are going to spawn it, make sure it's the right situation!