It's very common for users to ask questions which require clarification, elaboration, an example etc. More experienced users often comment on the question, asking for just that. Then the OPs comment, drawing previous commenters' attention to their amendments to the question; and so on.
Thus it is often the case that after 3–4 such rounds, the question is now in good shape, but it has 5–10 comments which only regard previous editions of the question and are entirely useless and confusing to future readers.
What I personally do sometimes is add more comments, asking previous commenters to remove all their comments, promising to do the same, then doing the same when they've removed everything. Surely there's some less cumbersome mechanism for all this?
One possible idea — which is not the reason I'm asking this question, just a possibility — might be the following: When editing a question, allow the OP (and/or the user reviewing the edit) to indicate which comments have been obsoleted by the edit. The user approving the edit (either the OP if s/he has enough reputation or the reviewer) also decides which comments to declare obsolete; and these will be removed. Possibly, with comment removal, commenting users can be notified about the edit.
(Maybe this should be a feature request, I'm not sure.)