Some manuscripts aren’t ready for the line-level involvement of developmental editing. If big pieces of your manuscript are apt to change — if there are major plot points that need to be addressed, or if there are a few darlings to kill off — a manuscript assessment is a better choice (and usually a cheaper one).
Following a structured editorial process, starting with an edit letter can also save you time and money for when you move on to developmental editing and proofreading. If you ask an editor to spend time on a full content edit of a manuscript that hasn’t yet had an editorial assessment, much more time will need to be spent on structural changes that could have been sorted out at an earlier stage.