The priority of this URL relative to other URLs on your site. Valid values range from 0.0 to 1.0. The priority value has no effect on your pages compared to pages on other sites, and only lets the search engines know which of your pages you deem most important so they can order the crawl of your pages in the way you would most like.
The default priority of a page is 0.5.
In my opinion, I see no need for having this as Google Bot digs through everything in on your website. Sometimes I find they even ignore robots.txt but not always.
Valid values range of URL priority between 0.0 to 1.0. This value doesn't have an impact on your pages in comparison to pages on other sites and simply lets the search engines know which of the pages you deem most important so they can purchase the crawl of your pages in the way you would most like.The standard priority of the site is 0.5.