This affects 2 things, both are only visual aspects:
1. Size of selection circle with green/yellow/red color (the circle that you can see under the unit when you select it or move your mouse on it)
2. Horizontal size of HP bar.
Using big value allows you to remove selection circle out of area of camera, so it looks like it's not there.
Using negative value allows you to hide hp bar but it also breaks selection circle so parts of it hides under the terrain.
I need to know more details. If you have Hero Selector and there are some creeps in the background and you don't want to see their hp bars this is proper way. After selecting Hero you need to change selection scale to normal value so everything looks normal again.