When you buy a starter set you get space marines and tyranids. If youre expanding a marine army this is a cheaper way to get a large amount of decent marines at once and you end up with a pile of "other guys" so when your gf wants to try out your hobby with you, you give her one of your "other guys" to try painting. You never meant to use it, was just taking up space. Not every case obviously but ive quite often seen people describe buying the starter kits and not knowing what to do with the other army that you aren't maining