The answer to this question depends on the state of your manuscript. Developmental editing is useful if you want to get in-depth feedback on the structure, thematic elements, characters, and overall story arc of your work. It's especially helpful if you have a first draft which may need some reorganization, or you're struggling with pacing and structure while writing.
That said, you may want to consider developmental editing even if your manuscript is in a more polished form. A developmental editor can help you strengthen weaker elements of your story, catch any plot holes, and hone characters and themes to create a stronger narrative.
In the end, it's up to you. If you don't have a completed draft and are looking for some guidance, developmental editing might be a great choice for you. If you have a finished work and feel like you have the editing basics down, you may not need or want a developmental editor.
