Outside perspective. Market knowledge. Experience with the structure and requirements of a story, especially a long story. Contacts. An indifference to the

Outside perspective. Market knowledge. Experience with the structure and requirements of a story, especially a long story. Contacts. An indifference to the 'darlings' of your sentences. They'll kill without mercy. Self-editing is completely doable, and some are very capable, but a professional editor is just that: a professional.