@Liedral at low cost and effort, he can easily profit from it compared to hunting. Also, he's losing potential profit by giving the guild parts (bones, organs, etc) he doesn't need for cooking. They did say in a few chapters before that some testicles of a beast is so in high demand because it's positive effects to a male sexual vitality which in turn have a pretty high market profit.
It also gives him control on prices on certain commodities which makes him very powerful. (Sadly, he hasn't reached that conclusion yet)
Edit: Fixed some grammar error