For many character classes it is often important/necessary to have different sets of items and armour for different tasks/roles, particularly for hybrid classes.
This mod greatly assists with this by allowing the creation of player-defined sets and switch between them easily, as well as a number of automatic sets - such as the ability to automatically equip a different set when switching warrior stances, or automatically equip your "carrot on a stick" or "riding crop" trinket when mounting.
Personally, I have found this mod to be invaluable for both my Holy Paladin (for whom I have created my own 3 sets of gear - Solo, Healing and Tanking), and my Warrior (on whom I use the 3 automatic stance slots).
Blizzard have strongly suggested that similar functionality may/will be added to the standard WoW UI in the future, but until they do this mod is a must for many.
Check out the mod yourself
here