In the Google+ hangout this morning a typical question around why isn't Google showing rich snippets for my site when the tool shows everything is valid came up. A typical question and we know Google doesn't show rich snippets for many sites because it doesn't reach a quality threshold.
But one additional question was added, that he thought that maybe his XML sitemap file being broken was the culprit. But John Mueller said that is not the case, at the 3:32 mark John said that although Sitemap files are great for finding new and updated content, once Google finds it, it doesn't help with showing up for rich snippets.
John said:
Obviously everyone wants to have great sitemaps and they're definitely useful for us for crawling new and updated content. But if we're already crawling and indexing your content and just not showing it the way that you'd like then the sitemap file isn’t going to be the one that's that's the reason for it.
Here is the video embed:
Forum discussion at Google+.