The application date is important because it determines which prior art will be used to evaluate your invention's patentability, and heavily influences your right to assert your patent later on. First-time inventors usually have no idea that this first step is so important. Utility patents also require an often rigorous examination by a patent examiner, who usually requests changes to the claims, and possibly to the specification, before allowing a patent on the invention. Design patents require only a drawing for a claim, and no textual descriptions. Design patents undergo almost no examination, and are therefore allowed quickly.
Many inventors suffer dearly for attempting to simplify the application process.
One of our physician-clients invented a new type of catheter for use in hemodialysis. Strapped for cash, and unable to pay the few thousand dollars necessary for utility patent applications, he applied instead for design patents on his catheter. Months afterward, with financial assistance from his licensee, our client did convert his design patents into utility applications. They also said it was unheard for a design application to continue into a utility application. While, in most cases, the last statement is true, in this case, we did persuade the patent examiner to give us the benefit of the earlier date because we pointed out that the drawing, together with his design patent's one sentence, really conveyed enough information so an expert in the field could build the catheter.