Games : How exactly do Code Generators work (if at all)?

So, looking online for help about a code that wasn't working for a free online trial, I noticed some sites claiming to have the ability to auto-generate these codes. Does anyone have experience with these? I am assuming since it is too good to be true (getting constant online trials/ free codes for DLC type items), that there must be something fishy going on.

So what's really the deal with these code generators? Anyone have an experience/warning they'd like to share?