For older editions, they make it possible for people to get older products that they couldn't afford as kids but can now as an employed adult - without requring a new and probably too expensive print run for the publisher.
Cut and paste. I can crib out the stats for a creature in the Pathfinder Bestiary and modify it for my game. Most of the stat block work is done.
Putting the PDFs on the iPad makes it a lot easier to tote plenty of sources around without breaking a back lugging the milk crate full of heavy paper.
A well-done PDF, like the Pathfinder Core book, has links so when an spell-like ability is referenced, I can use the link to go right to the spell description. It rocks.
With a PDF adventures, I can run adventures off a screen without breaking the binding of my print editions if the adventure takes a fairly long time to complete.