Self-publishing does not mean pure do-it-yourself publishing. It means simply that an author has decided to go to market without involving a traditional publisher. There are many good reasons why an author makes such a decision. One reason may be an adamant wish to dispense with all the services a publisher provides: cover and book design; editing and proofreading; formatting and typesetting; and marketing. Independent authors certainly have greater control over the publishing process, and this is undoubtedly an attraction of self-publishing. But only a complete control freak (or someone aspiring to imitate William Blake) would attempt to do everything without any outside help. There may be artistic merit and personal fulfilment in such a purist endeavour—and both are noble and admirable—but there is unlikely to be any commercial merit.
In a 2013-14 survey of self-published authors, there was a clear correlation between author earnings and their use of author services. The authors who earned the highest incomes were also the most likely to have paid for cover art, editing and marketing. This does not prove that author services lead to greater sales, since such services can vary considerably in their quality. But it does suggest a relationship between higher sales and greater professionalism. In particular, self-publishing writers with an income of more than $10,000 were more likely than not to pay for cover design and professional editing.
Intuitively, this seems right. Even with great marketing, a poorly produced book, riddled with errors, sloppy grammar, unclear syntax and a chaotic structure, and topped off with an amateurish cover, has little chance of selling well. The bad reviews and a poor word-of-mouth reputation that follow are likely to prove fatal to sales.
For anyone who regards writing and publishing as, at least in part, a means to make a living, the sensible business decision is to invest in the expertise of others to accomplish at least some of the tasks essential for a successful publication. Not all author services are good, and some seem designed to rip off independent authors. But the good ones are all about adding value to an author’s book. Choosing to add that value cannot guarantee good sales; it can do no more than improve the chance of them. But choosing not to add that value almost certainly closes off that chance.