What is Java used for? What would I need it for?

http://en.wikipedia.org/wiki/Java_(programming_language)

Unless you are creating your own applications you don't need it. Very few web sites use Java, so you can safely turn it off in Safari Preferences/Security.

But you do need Javascript, which is something entirely different.

Disable Java in your Browser settings, not JavaScript.

http://support.apple.com/kb/HT5241?viewlocale=en_US

http://support.google.com/chrome/bin/answer.py?hl=en-GB&answer=142064

http://support.mozilla.org/en-US/kb/How%20to%20turn%20off%20Java%20applets