IS an XML sitemap really a necessity??

Good day, ppl....

I was wondering if the absence of an XML sitemap will adversely affect SERPs.

Specifically, if a page has already been indexed by Google, and maybe even doing well with keyword SERPs, all without the use of a sitemap, will the inclusion of a sitemap do anything to boost rankings for keywords at that point?

I've always read that a sitemap is essential. But beyond letting search engines easily discover where pages are located (which they find anyway by crawling) how does this help anything?

Yes Xml Sitemap's are very necessary in case your website is very large, has very complex structure,contain rich ajax, or flash content's
for small website i don't think it is much useful.

The last time I submitted a sitemap was about 3 years ago... I don't bother with that stuff anymore and to be honest I didn't notice any changes.
Once search engines find your website, i believe they are smart enough to figure it on their own (if you have good site structure).
so if you have rankings and indexed, why bother with sitemaps???

Sitemaps don't affect rankings in any way, but it's good to have sitemap because the bots can find your content more easily