Originally Posted by rjd3282
It protects your dies from being scratched by dirty cases.
Exactly, it cleans up the brass so that fouling doesn't get in your dies. It not only cleans but smooths the brass also. This is an important step in the process for most loaders. With a tumbler it doesn't take much effort, just toss the brass in there, turn it on and wait a while. If my rifle brass gets really fouled I will even use a polish and rub them down by hand a little before putting them in the tumbler.