During the application examination process, a search of prior patents and published applications ("the prior art") is conducted by the USPTO to determine whether your design is truly novel. There are of course advantages to having a search conducted before or while preparing your application. This will allow you to determine if someone else has already patented your design.
Also, if you've had a search conducted, you can be prepared for prior art which the USPTO may bring up during examination. You can conduct a search for prior art yourself or by using a professional search service, such as LegalZoom. LegalZoom's comprehensive patent search service covers the relevant prior art in your field of invention.
Design patent applications require far less description than utility patent applications. The USPTO considers drawings to be the best "description" of a design. That's why it is relatively difficult to conduct a design patent search using keywords as you would for a utility patent search. However, a LegalZoom design patent search can help you cover the vast amount of existing patented designs that may be relevant to your application.