Performance: This is the in-memory part of the equation. Sorting, storing and retrieving specific in-use data entirely from memory rather than from disks makes all processes substantially quicker.
Cost: Hard disks cost less than memory, so some of the money saved can be used to add more memory to improve performance.
Persistence: Because RAM chips cannot get close to the storage density of a hard drive, disks are still used to store data needed for later use. This ensures that they are not lost in the event of a power outage.
