Some universities have a requirement that their doctoral candidates must have their dissertation proofread and edited before final approval is granted. Whether you are considering dissertation proofreading because you have to or because you want to make sure your dissertation is compliant with both citation style rules and your university’s requirements, having your dissertation proofread and edited will result in a final product that reflects the huge effort you have already put into it.
Think of it this way: You’ve been looking at this paper for so long that even obvious mistakes might not be so obvious to you anymore. If your Chair and committee members have also been reading your various renditions, they might not be seeing errors in sentence-structure, spelling, and grammar, in addition to citation or university style requirements. A fresh set of eyes—in the form of an editor who is seeing your paper for the first time—will pick up issues that have become invisible to you.