It stresses your RAM through varrying computations and transfers, then it double checks it to see if there are any corrupted RAM modules. It can also be used to stress test a CPU, and CPU/RAM together. It's not a 3D benchmark. If you want to really stress it that way, go to 3Dmark or some similar program.
It's more of a logical stress test, than a graphical muscle one.
Other than that, folks use it to bake in new builds to make sure there's no faulty problems with the CPU, motherboard, or RAM.
I've got a bootable copy myself. It's a handy tool.