The practical reason why people want a case for an iPad or reader is to protect the finish on a fairly expensive piece of technology. One scratch and the shiny surface will reveal black plastic. It is the difference between something that looks new and something that might be used. If the surface is already banged then a cover can conceal the fact.
People will also ask themselves if they should buy one or a few or a dozen or more. Most people get custom covers because they already collect them and might already accessories with stickers or markers or personal etchings. Someone not afraid to spend the money will have it factory made, because there is only so much a you can do to customize.
It is not cheap to have a lot of custom made covers ordered and delivered. The cost of manufacture will be much higher than the cost of shipping, since covers are only lightweight plastic. Some people want a cover for every month and every attitude. It is something of a fashion statement to be able to switch out on a whim.
It boils down to personal taste, although a single cover will do its duty for several years and by that time the next version of the unit will come out. Having a few can be a great backup if the cover ever breaks and some variation is preferred by almost everyone.
The people most likely to benefit from large orders is companies or teams who wish to have a uniform package. Having the organization logo on a cover can be a way to build team spirit and also to improve profile by investing into appearance. Any image, especially on high tech gadgetry, suggests a company is established and has resources.