1 ounce vs. 2 ounce copper weight PCBs

When you want your etch to pass more current before burning up traces? When you want relative voltage differences on the same plane across the board to be better? When you're using the etch itself for heat sinking, or as assistant to an additional heat sink?
 
In the "old days", manufacturers used to actually run solder down any heavy current traces for re-enforcement.
And in vintage transistor radios, and some electronics, the whole board's traces were coated with solder.
They stood the test of time, didn't they?