Hi
If you want google to know about changes to your pages before its bot comes around to crawl your site, sitemaps is the way to go. It really depends on how fast you want your site to be indexed and for google to see any new pages or changes to existing pages.
I use it just for the main purpose of making sure google has a good representation of what pages my sites do have. I've seen sites where google crawls every page in the sitemap after I've added a sitemap, and I've seen googlebot simply just grab the sitemap and not crawl the site.
I am one for the sitemaps
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