Professionally prepared drawings are the norm for any building works. Statutory Authorities have minimum requirements (i.e. drawings drawn to scale, details etc) on the presentation of your proposals. Contractors also require details of the proposals in a format that is easily understood and for which they can provide you with actuate costs. If the drawings do not show sufficient information the contractor may then guess what you require therefore there is an element of risk and it will cost you more.
Therefore, the design process demands good quality drawings that are essential for any building works.