If you are asked to say your current position to an information service before you reach your next reporting point, the information is going to be something like "3 miles north of X town, 1300 ft on QNH 995 HPa". That is enough information to provide basic situational awareness to other aircraft, giving them a direction and height to give the Mk1 eyeball a direction to look in. The information provided by PCAS (or TCAS) is as accurate, providing a basic help to situational awareness.
Certainly you don't know whether thats the correct aircraft, and the direction may well be off by 30deg, but it gives you a starting point for the MK1 eyeball. That is useful information. Why not share it?