I assume that you are talking about sitemaps and it is basically a way for sites to try to control how often the site is crawled by bots.

It is a "suggestion" only and not a mandate. Search engines will decide for themselves whether or not your site needs to be crawled more frequently or not, based upon how often the pages on the site change. What you suggest to them is, in large part, ignored once they get a feel for how often things change.

Priority and Frequency are not as important as they used to be and are pretty much ignored. Timestamps (lastmod), however, might be more important. You can get a pretty good understanding of Sitemaps from this article