It gives you a good glance how likely it is that something is a threat - if an overwhelming majority of AV products detect it, it's quite likely that it's bad, and if only a few products find it, it's either very new or really just a bunch of false positives. Like many things, there's no simple "yes" or "no" answer, thus interpreting Virustotal output still requires some literacy on that topic. The site can also help software vendors to find which AV products reject their software, so that they can file false-positive requests to those AV vendors.