Sitemap is only use for getting the pages indexed faster.i don't think so that it will affect google ranking. Accept Payments Online
Sitemap is important to the website, it helps you to identity which link are broken were google cannot crawl. It really helps who want to rank faster.
does anyone know if it is a good thing to block pages on the sitemap? Like, let's say, if one of your page is not optimized...
Basically the site map help any website to get the faster index form the search engines as they find all the pages at one place.But you do not have a sitemap does not mean that you will not get the crawl.
If you use a real site on the website, it may help a little, because it's a internal link. But a sitemap uploaded in GWMT is only to get the site indexed faster, it will not help to the rankings. By the way don't use these sitemaps, if the sites got indexed, you cant track where to put more work in you site.
Sitemap is only useful to present our webpages on search engines and get indexed by them. There is no use for optimization on SERP
Search engines should see all the pages that you want them to see. The more pages that they index from you, the more trust your site gains. It only means that your website has more information to offer. Making sure the search engine spiders get to crawl all the stuff they need to crawl from your website is the exact purpose of a Sitemap. It’s not for navigation, it’s not for internal linking. It’s for the search engine spiders. It won’t greatly boost your on-site SEO, but it would definitely help.