There are thousands of Addons available for WoW which can be installed manually or via clients like curse client.
Using addon is really helpful as it helps to do quests and level profession skills.
But you can play without addon without any problem as I dont use addon any more, But I am not having any problem.
In case you are new to wow using addon might helps. As it will help you to pinpoint exact location where is your desired mob or quest is, And I assure for a new player searching huge area of the map is not easy.
So for new guys my advice will be to download curse client (addon finder if WoW)
Than download following addons
- Quest helper
- One Bag
- Pet Control Addon (anyone which suits you)
There are thousands of addon available...just search for it, n use ...remember it depend on players playing style which addon he or she use...Lyk i hardly use addons...
Addons can be enable/disable in character selection menu of WoW... As well you can configure addons in game menu/Interface option