Unfortunately most instructors are not too keen to give ground instruction, possibly because they seldom get paid for it! There is a shortage of enthusiastic ground instructors at PPL level and the AOPA ground instructor certificate is a good way to train non flight instructors to teach the theoretical subjects.
No qualification is needed to teach PPL groundschool but, you need to gain the knowledge somewhere and the AOPA course is conducted by the same FIC instructors who teach flight instructors. The time spent on such a course can subsequently be counted towards a Flight Instructor rating.