Autoprefixer doesn't just add prefixes, it translates the the syntax to browser compatible code as well, for example:

    a {
        display: flex;
    }

Translates to:

    a {
       display: -webkit-box;
       display: -webkit-flex;
       display: -ms-flexbox;
       display: flex;
    }

It supports the 27 differences between the various browsers.