Formerly as a D&D 3.X dungeonmaster, I used to cart a crate full of books around to whichever friend's house was hosting. I liked to have all my best books at my fingertips, although it was a pain to have to load and unload the car with them.
I knew that Paizo offered its books in PDF format but I wasn't really sure if it would work very well to run a campaign with all the books in electronic format. But I bought the core book in PDF anyway and stuck it on my smartphone as an "electronic backup" because, after all, every night both my hardcopy rulebooks would end up getting borrowed by this or that player and I would never really get to use them. And so it went.
I got used to looking up things on my phone, and later my laptop when I transferred the PDF to it. I decided it was pretty nice to have it not taking up space on the game table when I had to have my NPC sheets, maps, game notes, and whatnot all over the place. So then I invested in other PDF versions while keeping up with my hardcover collection, Ultimate Magic, Adventurer's Armory, Ultimate Combat, and so forth, and saw that I could lend out my hardcovers freely for the players' use while I still had access to everything on my laptop.
Finally, I began picking up PDFs without the "hardcover backups" since I didn't have the funds to get both, and considered how expensive it was to keep up to date in both book formats. And I realized after I got Bestiary 3 on PDF, there wasn't any real point to getting the hardcopy because I was the only one who really needed to be looking at it. So now I am only buying books in PDF format, and would only buy hardcopy for whatever serves as the core rulebook for players to use.