You can view ebooks in your mobile devices/computers. But you can't do that with regular books. I think that is the main difference.
Edit: However, one important thing to mention is that there are ebook-to-regular book converter devices that you can buy from stores like amazon/ebay/ali express. They are called printers. regular book-to-ebook converters are called scanners.
Edit 2: To mathematically define the relationship between those two devices, if printers are defined by f(x), then scanners can be f-1 (inverse function)
Just an alternative to those who can't afford to buy a real book. Imagine using a cheap chinese duplicate phone when someone cannot afford to buy a real iPhone.