Sitemaps only help the googlebot understand the link structure and contents of your site. It doesn't actually do any of the crawling or directly help indexing. It indirectly helps by telling the bot more about your site.
Atleast this is from all that I've ever read about SEO and indexing, but always curious to hear from experts.
I don't think he said anything about SEO really. He just wants a search for "How do I foo a bar" to find the "How do I foo a bar" question on his site. Since there are no links to that question, the only way for Google to find it is via a site map. Totally reasonable.
Atleast this is from all that I've ever read about SEO and indexing, but always curious to hear from experts.