Why do people buy ebooks?

Instant gratification is part of the reason.

When you buy an eBook, you can (usually) download and read the book right away. If you're looking for answers for something, do you want the answers now, or in 3-5 days?

Another benefit may be that the eBook is written in layman's terms, and may not be as difficult to understand as a "professional" book written on the matter.

Consider too that some people may have the knowledge but may not be the best of writers. Or they may not have the patience to go from publisher to publisher looking for someone to publish their book. A publisher may lend an added level of credibility to a book, but that also adds to the delivery time and keeps out a number of otherwise knowledgeable people. An expert may not have the patience to send out queries, wait for responses, send out sample chapters, etc. That all takes time and, for some topics, time is of the essence.

With an eBook, there is perhaps a higher risk of getting a "bad" book (of course, there are a number of "bad" books that get published by publishing houses too!), but the information may be more timely. And the information can reach you sooner. The content of the book might also be too small a niche for any major publisher to deal with, so it might not otherwise have even been published, even if the author was top-notch and an expert in his or her field.

So, ePublishing also allows for a wider variety of topics, and especially those in smaller niches, than publishing house would even bother with.