Some questions get marked as problematic for one reason or another. Flagged posts may be deleted, put on hold, or edited.
Flagging a post is basically calling attention to the post being problematic. This means it either needs to be deleted or modified.
Kinds of postings that are a problem: A spam message. A question that is outside the scope of the site. A posting that is hostile or insulting. Keeping spam and hostile messages and outside of scope messages, makes the site more appealing to those who have legitimate questions (and those who wish to help them) by avoiding wasting their time with irrelevancies. 6 flags will delete these questions. This allows a large percentage of the community to quickly remove postings that should vanish since even a low reputation user may flag them.
It is a pretty straightforward idea that no one enjoys answering the same question repeatedly, and many copies of the same question will dilute the value of the site to someone looking for answers. So another valid reason for flagging a question is that it is a duplicate and has already been answered. Some questions are not narrow focus enough to allow for a concise answer. These will be flagged as too broad.
A question that calls for an opinion could very easily be considered problematic as well: "What is your favorite approach to X?" does not really educate the asker, and calls for interpretation on the part of the person answering.
As a persons reutation increases, they have greater privileges to edit postings. Flagged posts may be edited and/or deleted, (and the user who created them warned if need be) by a moderator or a user with sufficiently high reputation.