Even though there are standards, some browsers still like to use their own standard, like … IE (surprise!?) Furthermore, even different versions of IE will also render your site differently!
iI addition, there are million browsers and platforms out there. You can’t install everything nor have all those platforms to check your website compatibility.
To check what your site look like in other browsers and platforms, use this tool to ensure your site is more or less consistent or not too far out. Note: be patient, it may take a while to get screenshots.