For awhile things were going along quite well and everybody was happy. Then, one day, the owner of a local theater read a novel and decided it would make an interesting play. In the early days of horse and buggy when books were delivered only to a small area, local rights were all one needed. The relationship between all parties was going along quite well for most of the 20th century. As recently as 1995, the Writers Market didn't even show a listing for ebook rights.
Electronic rights can cover a lot of territory.
Take this article for example. It's being read on your computer screen, but you can also download it to your hard drive, make a print copy, or several print copies, and send it out across the world in an instant. And the author, me, wouldn't receive one dime for any of it. However, before you hit that button, check the top of this page, and you'll see a little symbol. Which means if I catch you making money off my work, I'll see you in court.