When you put a disc into your PlayStation 4 before you can start playing, it will begin to save large chunks of that disc to the hard drive. This isn’t optional, this is mandatory for the game to be played. The system will begin caching the data immediately and will you can begin playing the game, but it will continue until all the stat has been transferred.
The amount of date saved to the hard drive will vary per game. For instance, PS4 launch title Knack will require 37GB of space to play. Cerny has said that users would only have to wait 10 seconds before playing as the game will stream the rest of the content to the hard drive during the session.
Call of Duty: Ghosts is going to require 49GB of space and Killzone Shadow Fall 45GB of space. It isn’t going to take long for the 500GB hard drive to fill up. And that doesn’t include ant downloadable titles or DLC. the cached date will stay on the console until the player deletes it, presumably to clear some space for a new game.
The reason for this, is that the PlayStation 4 is not designed to read the game off of the disc, but the hard drive. And the reason for that is the laws of physics Mark Cerny explained. The PlayStation 4’s Blu-Ray drive may be 3 times faster than the PS3’s and have about 6 times the memory, but it is still faster to read data off of the hard drive. Given the amount of data needed for the next gen games to play, the only reasonable answer was for the system to read them off of the hard drive.