Why an eBook instead of a traditional book?
Firstly, don’t think of it as an either-or decision. There is nothing stopping you publishing your printed work in digital format; equally there is nothing stopping you publishing your digital work in classic print format.
In fact, there may be situations where, frankly, viewing your work on a tablet or computer simply doesn’t convey the message you want to give your readers. For example, many fine art books, in my opinion, simply can’t be made into digital books because only a bi print allows you to see the full picture along with the detail in an instant. Zooming in simply doesn’t do it. The same goes for some works on architecture, and many books with large format photography (and I say this despite the fact that many books use photography to highlight the text).
So, to rephrase the question again, why an eBook at all?
The main reason today is that the market (i.e. readers like you and me) are increasingly buying them. They buy them through big retailers like Amazon and Apple, as well as the various online versions of traditional bookstores like Barnes & Noble, Waterstone’s etc. If you don’t publish a digital version of your book, then you are missing out on that market. That’s reason enough.