Breaks ground loops at 4 cable method
I use a Randall RH-50T amp with a Boss GT-8 effects unit via the 4 cable method.
This lead to a very annoying ground loop between the guitar input and the effects loop.
With this handy device the humming stopped immediately.
Also comes in very handy on stage if any problems with humming ground loops suddenly appear.
Although I have to admit that the signal experiences a little decrease in volume when you send it through the HD400. But not by very much, so it doesn't really count as a downside.
Especially when losing that annoying hum on an instant is the big upside.