From StrategyWiki, the free strategy guide and walkthrough wiki

In World of Warcraft, skinning allows you to skin beasts to gain different qualities of skin, which can be made into leather by the leather working profession. In addition to leather you can also skin some mobs and loot scales This makes the professions of leather working and skinning a good combination especially if you’re class is restricted in what kind of armor they can wear. Shaman for example can only wear leather or cloth armor up to a certain level so creating your own armor and applying reinforcements will provide you with on par armor through the earlier stages of the game. Another advantage to skinning is that you skin the beasts you would have to kill anyway. There is no need to go out of youre way to find herbs to pick or veins to mine. In addition to creating your own armor you can pick up some extra money from just selling the leather. One quest in Thunder Bluff only requires 12 pieces of leather to complete and rewards with something like 650 XP.