For the identical reason patents and other references are cited in utility applications. Design patents are searched and examined by patent examiners. What they turn up in the search ends up cited at the beginning of a granted patent. Also, the applicant is required to notify the examiner (on an IDS form) of everything they know of that might be relevant to the examination. Those references get cited at the beginning of a patent publication.
Design patents really are subject to the requirements of novelty and non-obviousness that utility patents are. The relevant sections 35 USC 102 and 103 make no distinction between the three types of US patents.