If you're going to self-publish, it helps to be able to create your own ad images and book covers. Even if you want to pay for a fancy cover for your main title there are going to be times when you want to have a loss leader short story or where you want to have a banner image for your Facebook page. And it isn't going to make sense to pay a designer for that type of work. (Not unless you're richer than most authors or doing really, really well.) So...