Plants normally have leaves for the purpose of food production. Leaves are also used for transpiration and absorption of sunlight. Other functions of leaves are floral induction as well as food storage and provision of physical support.
1 Additional Answer
The main reason that plants have leaves is for their surface area. Plants need energy from the sun to carry out photosynthesis, so the more surface area they have the more energy they can absorb. You can find more information here: http://www.backyardnature.net/photosyn.htm