important if google decides it is important to where you rank in the serps
look at it this way, your site makes it to page #1 for a keyword, but your site is showing a high bounce rate when google sends a searcher to you for that keyword, google may then assume that your site is not relevant to the surfer that searched and clicked, so they decide to bump you down a page or two and send you less traffic for that keyword
meanwhile, a site like pornhub gives the surfer all they want for free, so everyone that google sends to pornhub bookmarks, stays and enjoys themselves, and google's algorithm then says, wow 0% bounce rate, this site deserves to be #1 in the serps