are sitemaps necessary?

[suwunk]

Sitemaps are particularly helpful if:

Your site has dynamic content.
Your site has pages that aren't easily discovered by Googlebot during the crawl process—for example, pages featuring rich AJAX or images.
Your site is new and has few links to it. (Googlebot crawls the web by following links from one page to another, so if your site isn't well linked, it may be hard for us to discover it.)
Your site has a large archive of content pages that are not well linked to each other, or are not linked at all.

About Sitemaps - Webmaster Tools Help
You should not use Sitemaps if your site is a private website, for example, for privacy from search engine results

[PotsNPans]

If you have a website, do you have to set up a sitemap? Is there any reason why you wouldn't want to?