Yeah. I have seen this too. You are not alone.
I don't see any reason to do this. In fact it is redundant isn't it? And since sitemaps are designed to inform search engine about resources to be indexed and sitemaps are not indexed, it seems rather pointless. It was always a silly thing to do. You will see silly things all over the place especially in particular to the web and SEO so please take all of what you read with a serious dose of salt.
You did the right thing by asking.
Sitemaps have no effect on SEO in regard to site/page ranking and SERP placement. There is one performance metric that a sitemap can positively effect; whether a site is considered crawlable if there is a barrier that blocks the site from being crawled by the bot.
They are designed to inform search engines what resources that are available on any particular site. This is because there may be resources that are not linked, behind a login, or far too vast for timely discovery. End of story. If your site is smaller or medium sized, then search engines will likely prefer to just crawl your site. What they will do is read your sitemap, and compare it to their ability to crawl your site. If there is no difference and the site is small enough for natural discovery, then they will continue to crawl your site the old fashioned way. But still, from time to time they will compare your sitemap again to make sure that they are on track. If you suddenly expand your site, they may read from your sitemap. It is a good thing to do regardless, but not necessary for every site.