One thing I don't get, why did they have to launch a subsatellite to test the net even though they had a map of where most of the debris is located.

How the future will look like? I think the headlines will look like this;
"Elon Musk finds a way to collect all spacejunk."
The launch of Space X's Operation Junk Wars (Collect Junk and Recycle on the spot, transport it back
to earth).

I'm telling ya, that car he shot up there is remapping the location of debris...