It can be useful to have a range of signals. Flagging is a rather hard-line "this is not appropriate" mark that many are unwilling to make on borderline things

It can be useful to have a range of signals. Flagging is a rather hard-line "this is not appropriate" mark that many are unwilling to make on borderline things, e.g. when only somewhat misleading or partially incorrect. Plus it requires clicking into the individual message, so you are required to see the contents twice, and can't do it quickly. That friction alone slows down using it on any softer or higher volume cases.