I think a very useful case for the EPUB eBook format is for developing documentation. Documentation is more than telling people how to use computers.
PDF is good and useful, but one problem with PDF is that it is limited to a computer or paper and there are plenty of instances where that is less than ideal. For example, an owner's manual for a car. Who wants to carry around a huge book in their car or have to drag their laptop to their front seat to be able to figure out how to program their navigation system. With EPUB you could put your owner's manual in your smart phone in a way that has a table of contents and is searchable.