... what is the use for this? ...
I’ve used RDF to search for downed aircraft, and to track
down interference (like noise sources, stuck microphones,
a cat sleeping on someone’s CW key, jammers, etc.).
I’ve nearly had to do it a couple times to find people or
vehicles who got lost, but had a radio with them.
Climbers on Mt. Hood often carry radio beacons so they
can be located in an emergency.
Mostly I do it for fun. But it takes skill, regardless of your
equipment, and if you don’t do it regularly, then you may
not be ready to do it when needed for some other reason.
So lets say its for the criminal element, but they aren't using 2 way radio or are they?
Some of them certainly are.
Cell phones leave an identifiable trail. If you have
a couple lookouts outside while doing something
nefarious in a building, that’s a common way to
keep in touch. Or keeping in touch with a rural
hide-out, or coordinating activities over a wide
area. Sure, internet is handy when it is available,
but mobile service still usually depends on a
connection to a cellular network, which leaves
the same trail as a cell phone does.
I knew one game warden who was tracking
teams of poachers. In inland parts of the country,
marine HTs were popular, as the radios were
easily available (at Radio Shack, for example)
and the frequencies weren’t otherwise used locally.
These days, wideband radios are relatively cheap
(even for HF), and folks can set themselves up
on a quiet frequency where they aren’t likely
to be noticed.