I believe the idea is that you use on when motor mounts are simply not strong enough to tie the motor to the chassis. It looks like a big plate of steel that bolts to the front of the motor.
Pretty much as 89 said...if you have too much power for the motor mounts to handle or you are doing a big block swap, you will need a plate. Most smaller motors would never need one. Its not so much an avantage, as it is a necessity at that level of power.