For a writer to improve what isn’t working in their manuscript, they need to know what is. Different professionals can help you in different ways.
While beta readers are intended to give you their perspective of your work as a reader, a developmental editor is intended to give you skilled insight into what isn’t working and will offer suggestions and solutions as you revise your draft.
Said another way:
A good beta reader will tell you what they liked about your story AND what they didn’t.
A developmental editor can tell you WHY things work or don’t and help brainstorm solutions for the problem areas.