I think that a big part of the reason for so many comma questions is that guidance for their use (on this site as well as elsewhere on the Internet, in popular books, etc.) ranges from "every comma should be justified by a specific reason" to "insert commas wherever you think they seem reasonable". (I'm paraphrasing, of course, but that's not too far off.) With such a variety of opinions, it's no wonder that people don't know what to do!
Perhaps we can try to create some "canonical" posts explaining the common uses of commas and the various opinions that authoritative sources offer for their use.