A slightly different way to call QC'ers would be beta testers.
It's all quality. Making sure that no typos slipped through, everything sounds fine, the game runs fine in English all the way, stuff like that. The Majikoi titles are loooong. It may sound easy to do some of the more basic stuff, but you still have to analyze thousands and thousands of lines, all while needing to know what is proper spelling and punctuation.