Comment by giantrobot

Mainframes aren't supercomputers. The point of a mainframe (anymore) is reliable transactions without downtime. They're not necessarily beasts at computation.

Supercomputers were and are beasts of not only computation but memory size and bandwidth. They're used for tasks where the computation is highly parallel but the memory is not. If you're doing nuclear physics or fluid dynamics every particle in a simulation has some influence on every other. The more particles and more state for each particle you can store and apply to every other particle makes for a more accurate simulation.

As SCs have improved in memory size and bandwidth simulations/modeling with them has gotten more accurate and more useful.