Why would my re have me take an hcg trigger shot for an ovarian cyst?

Follicle: If they thought it was a follicle (growing an egg inside of it) rather than a benign cyst it may work to make you ovulate and therefore increase chance of pregnancy or get rid of the cyst. I would ask them about their rationale for doing this. Before you do treatments its best you understand why you are doing it rather than worry if its the right thing to do.

Dominant follicle: If it is for a dominant follicle, the trigger is to make you ovulate at the right time. If it is for another reason you have to ask the re, no one else can tell you his/her reasoning.