You trade a "throw-away" Pokemon like Starly or Bidoof. You pretty much trade a Pokemon you don't want, in return for one you want.
Pretty much, I am offering a Level 5 Eevee for any Pokemon you don't want or have "worthless" multiples of. For example, if you bred a bunch of Ditto, you wouldn't trade me that because it can be rare in some trainers eyes. Bidoof and Starly aren't rare unless they have good egg moves. Or unless of course they are high leveled/ IV trained/ UV Trained.