Having an e-book cover gives you a graphic element to use when promoting your e-book, and it gives e-books more of a traditional product feel (more of a "product" than just a file, from a buyer's and marketing standpoint).
But how do you design and create one? I'm no expert in this area, so I'm just going to point you to a few resources that can get you started, along with a tip or two:
E-book Cover Tips
- Make sure your cover comes in multiple sizes for different uses.
- Make sure the type on your cover is readable (at least the main title) even in it's smallest size option.
- EbookTemplateSource.com - free
- EbookCovers.us - free and paid options
The basic idea is that you start out with a flat image or text, and you stretch / skew it onto a pre-made cover template. It's how I did my simple one for Press Releases Made Easy, and it was pretty easy to do. You'll need image-editing software that allows you to manipulate the images in that way.
If you don't want to design your own e-books, you may want to try software like the instant cover creator (I haven't personally tried it), or you can always hire a cover designer (most I've seen will do an e-book cover for under $50).