Chris Evans of Architecting IT comments:

Given infinite resources and infinite budget, all data would be stored on the fastest media possible, because we know that storage is the major bottleneck in most IT infrastructure.  Unfortunately life isn’t that simple; businesses have budgets and are constrained in environmentals (floor space, power, cooling), plus staffing resources and of course with exponential growth, hardware costs grow with a similar profile.

This is a great overview of tiering from Chris. All the reasons why you would do it and how it’s changing.

Read more at: Understanding Tiered Storage