Developmental editors are concerned with the structure and content of your book. If your manuscript lacks focus, your DE will help you find the right direction to take (the “right” direction generally being the most marketable). This is where problems of inconsistent tone, an unclear audience, or an unidentified marketing niche often surface.
Developmental editors perform many of the same editing tasks as an acquisitions editor, but unlike AEs, whose time is split between editing and the business side of publishing, DEs tend to be able to give you more personal attention. If you have hired a DE on a freelance basis, this is undoubtedly true. Either way, you will find that good DEs are friendly, organized, valuable to your endeavor.