The answers is: depends. Well, rooting your phone means getting root level privileges on your android phone. Root is THE user on your droid system, the one who can change system setting, and possibly make your phone stop working ;)
If you are a "standard" user, you don't really need root level privileges on you phone. All the applications you need (Facebook, Twitter, messaging, and so on) will be available to install, and you'll not have to care too much about permissions, other than relating to your privacy: a malicious app will not have much chances to damage your phone.
If you are a "power" user... well... yes, you absolutely need root level access!There are some reasons to do that.
First of all, you'll gain the capability to install and use apps that need root level access. Think about Titanium Backup, a really must have app in your phone, or Apps2SD Wireless Tether (if you don't have built in your stock ROM).
Also, you'll be able to set your phone as you can set a linux box (almost), like editing the hosts file, or having apps that setup your resolv.conf in the way you like.
The side effect is that you really can mess up your phone, or let the applications gaining root level access do so. So be careful on what you do on your rooted phone.