Why have one? As far as I can see it offers only these benefits:
1. It's a platform for someone to take photos.
2. If the test plane has an avionics failure the chase plane can make the radio calls.
3. The chase plane can look out for traffic.
The reason why I'm asking is that a friend is going to make the first flight of his plane soon, and has asked me to fly chase for him. We have flown formation together numerous times and have briefed on that and other appropriate aspects of the flight.
I will not go in close enough to identify potential leaks, so that possible task isn't going to happen.
But if anyone can give me some advice, something to help make the flight safer or let me relieve the test pilot's workload, I'd be grateful.