Many author-publishers design their own book covers. And on any writing forum, you will find best-selling author-publishers discussing whether it is the biggest false economy those people make, potentially even more incorrect than not obtaining professional editing. Taken as a balancing act between cost (in time or money) and increased sales, there are good arguments in favour of spending the time writing and the money on a cover. However, I believe there are other less tangible outcomes than sales from a cover design; such that even designing a cover knowing it will never be used can be a useful step to take.
This article by Brooke Johnson shows, if an author has the skill to do it, they can produce a good cover. But Brooke freely admits it took time to make. Time which (especially with sites such as Fiverr offering custom covers in a few days starting at $5.00), many serious author-publishers argue would have produced more money if spent on writing the next book, polishing the current one, or some other writing task. And they might have a point when measuring actions by their direct impact on selling books.
However, there are several other benefits:
The Effort of Realisation Just as it is easy for others, based solely on the end product, to think writing a book is just putting down words, it is easy for authors to mistakenly think cover design is easy. Once you have tried to design a cover yourself, you know how much work is involved in creating one for you. So you have a realistic baseline when looking at a designers portfolio and fees.
The Value of Tweaking Most authors have a better understanding of how words work than how pictures work. By creating a cover you will, if you are lucky, gain a better understanding of visual art, or at least some examples of what doesn’t work. So, when you do hire a designer there will be fewer requests they try something you half-took from the cover of another book.
Common References A good cover catches the eye, reveals the genre, makes the right reader more interested, and lets the reader who will never enjoy it know it is not for them. All in a small rectangle of coloured patterns. Unless your cover designer has read your book, they will not know all the themes, characters, or even which part of the genre it is in. Designing your own cover forces you to distil thousands of words into a few search terms in a photographic database, or to find a moment that stands on its own without explanation.
Enough and No More There are several thrillers where I worked out the ending from the cover art. There are many books where the cover does not quite match the events or characters of the story. Even if you do not use the cover you design, you are the best person to know what needs to be shown or not shown.
Relaxation If you have some skill with an image manipulation package, you probably at least mildly enjoy being visually creative. Designing your own cover is a great way to take a rest from writing without your inner critic complaining you are being frivolous.
So, even if I decide to hire a designer for the cover of a future book, I will continue to design covers to gain a better perspective on what the designer needs to know to make the best cover.