1. Room in my house. I have a small house; the number of books that can fit inside is limited. We're a 6 person, homeschooling family, and my husband collects comic books. My books get the least space priority.
2. The ability to immediately start a new book when I finish a book, without having to carry around multiple books. I can also immediately check out a new book from the library without waiting for my husband to get home with the car, and then fighting the snow and sub-zero temperatures.
3. No-handed reading. With a dead-tree book, I have to hold it open. With an ereader, I can do something else with my hands, pausing only to press the "next page" button when necessary. I often use this to knit or crochet while reading.
4. The ability to switch between ebooks and audio books, or reading and using text-to-speech. I can continue the book while I have to cook, drive somewhere, etc.
5. Weight. My Kindle is much lighter than most dead tree books I read, making it much more comfortable to hold. I can read longer because my hands/wrists don't get tired.
6. Okay, I'll admit it... piracy. I buy a lot of ebooks, get ebooks from my local library and am subscribed to Kindle Unlimited, but I pirate books too. (Most often books I can't get electronic versions of legally.) Printing out a whole novel when you're paying for your own ink is not cost effective, though admittedly I did it a few times in the 90s using the free printing at the school library.