tedster - 6:47 pm on Sep 3, 2012 (gmt 0)

I see sitemaps as the easiest way to know roughly how many URLs you've got indexed, thanks to the Webmaster Tools report.

Another plus, when a site publishes a lot of new content on a regular basis, it helps to get that content into the index a bit faster. But no, you don't "need" them and they are not a ranking factor.