Yes, It really help a lot, if you are using xml site map and html site map. Both have their own importance, people can see the pages directly from the html site map. And xml sitemap helps to instruct the search engine to crawl more pages. Thanks Rob popkey
Well, don't have a sitemap and according to most of the replies, you "must" have it but there is no real reason to have it. My site is being indexed the next day I update it.
Sitemap leaves a healthy message to search engines about a website structure so it is highly recommended to use one for getting high rankings in Google
It works for indexing your webpages purpose, persoanlly I installed sitemap in my website and all my pages got indexed in 1 day.
For one of my brand new blogs, the sitemap did help a lot, for the search engines to index my content. I used a plug in for it, that basically puts in one page all content of the blog - but under the specific categories - under pages - all pages, and under posts all posts...so that really worked well, so I think just about any site should have a sitemap.