First of all, oil is basically used by plants as a store of energy, just as in animals. So generally fats aren't needed in plants, they can get by with carbos. BUT Fats and oils hold more energy than a given amount of carbohydrates - about double the energy.
Animals use oils and fats for the same reason.
Think of it as efficient energy storage. Then make the creep sleep on the couch. Cecil's Mailbag is researched and written by members of the Straight Dope Science Advisory Board, Cecil's online auxiliary. Inc. All rights reserved. No material contained in this site may be republished or reposted without express written permission.