Detangle with less breakage
Applying oil to your hair before washing it will help you detangle your hair before it gets wet in the shower. Wet hair breaks more easily than dry hair, so it's better to get through as many tangles as you can before you shampoo. After you wash your hair, you might not be able to tell that there's any oil left, but there will be a small amount left that will help you detangle your hair when you put the conditioner in.
Get more out of your conditioner
Most conditioners for dry hair are made with natural or synthetic oils. If your hair becomes too clean from a harsh shampoo, the oils in your conditioner are more likely to go down the drain when you do your final rinse. A pre-shampoo oil treatment will give the oils in your conditioner something to "hold on to". That means you'll get to experience more benefit from your conditioner.
Updated March 17, 2012