First, it shows your children you can be entertained without an electronic device. My oldest son is three and already an expert on the iPad and iPhone. It worries me how much he wants to use the device, and telling him he can't use it and the whole family is taking a timeout from electronics is important to his growth. He can draw while I read a book, for example.
Second, buying physical books at a bookstore is a great bonding experience. I love taking my little guys to the local Half Price Books which always has a great dedicated kids section.
These reasons aren't only for parents, of course, but as a parent I find having physical books present is important.