Sketching can generate many options very rapidly. Drafting prior to making a list of sketch alternatives can result in poorly conceived products.
Sketches are important communication tools.
A drafted drawing implies a more advanced state of planning which intimidates the client from making suggestions. They are afraid to make suggestions because it appears that they have already invested too much in the plan. This can result in inferior design and poor satisfaction with the end product. A sketch implies an early state of planning and designing which invites input and participation on the part of the client. The resulting product is more likely to be appreciated and understood by the client.