There are two that I can think of:
1) To solicit suggestions, information, feedback. For this I'd say that anything more than 20 comments becomes difficult to parse.
2) To engage readers in a debate -- in some cases, not even bothering to participate. For some this might be to write something inflammatory and watch the fireworks that follow. Here, there can be no limit to the number of comments or even the need to police the comments as Prof. Mankiw had been doing.
I would believe that most bloggers prefer (1) but occassionally lapse into (2). The comments on Marginal Revolution and Econbrowser are usually pretty good. I prefer those on Econbrowser.