Information LifeCycle Management (ILM) is a concept that says the most used data should be the fastest to access.  All but the most basic storage appliances will provide some sort of ILM that allows the most used (or more frequently changed) data to reside on the fastest level of storage in your organization.

Manufacturers vary on how they implement this, and how they automate this.  Integrating this functionality into your business lets you squeeze the last ounces of performance out of your IT investment.