Some sites deny access to visitors using certain browsers. Zealots wanting to force you to use "the best" browser or even "the best" operating system. But how can visitors read what they have to tell? The advantages of their proposed solution? Some people can't freely choose which browser or operating system they use. Or have other good reasons to use what they are using.
Also with some browsers it is possible to change the user agent setting so you can never be sure of actual type of browser the visitor uses.