Well the most obvious answer is to keep clean but it goes deeper than that. When they lick their fur it smooths it and makes it a better insulator. If their coat is ruffled it doesn't protect as well in the winter time but there is much more to it still. Coldness isn't the only problem overheating is too! When they lick themselves the saliva evaporates cooling the cat off. Lions and other great cats do this as well. Cats do not perspire like us because they do not have sweat glands so licking themselves is the only way to cool off on a hot day where ever they may be. A nifty thing also associated with the sun and cats. Have you ever noticed how they like to bathe in the Sun and lick themselves? When they are doing that they are actually tasting the Vitamin D from the Sun and that is how they get the Vitamin D that they need. If your cat is a nervous cat or agitated they groom more frequently this is called Displacement Grooming. Cats are also VERY Smell orientated like we are very site orientated. You ever noticed right after you pet it it starts licking that spot? You know why? It's because your scent has covered it's own and it doesn't like to smell like anyone other than itself. It is trying to Rebalance its scent!
Auto grooming is a set of actions done by your cat. The following sequence is usually how it goes.
1. Lick the lips
2. Lick side of one paw till wet
3. Rub cheek and face with said paw therefore cleaning that side of face
4. Continue with other side
5. Lick front legs and shoulders
6. Lick Flanks
7. Lick Genitals
8. Lick hind legs
9.Lick Tail From Base to Tip
If you interrupt your kitty in his/her grooming cycle they will pick it back up in the spot they left off. I observed Jolie once doing this!