Well, for one thing, there is a high risk that your cover is going to end up looking suspiciously like at least half a dozen others, as authors pick from the same limited stock of options. Secondly, it’s amazing how tiny things about the way a cover is put together can change how professional it looks. Most of us do it at an entirely unconscious level, but put two covers together and we will instinctively prefer the one that obeys all those hidden ‘rules’ about visual impact. Designers understand that.
So find a good designer, one who can realise your unique vision for your book. And shop around. Designers can be expensive – but they don’t have to be. There are some outstanding ones out there who are also very affordable.