http://www.jazz2online.com/d3/plus-readme.txt

to answer the question at least a little, although the above link answers them in much more detail than I can, no, you don't really need it for anything. Servers will largely take care of themselves and will kick you or put weird symbols by your name for not-readily-apparent reasons all on their own. It's more or less useless for single player, other than maybe the edit of the Generator event. MAJOR FEATURES are more than clearly covered in that link and if you have any questions about specific items (if they're not clear, for instance) then that would be an entirely respectful and reasonable question you should feel free to ask.