It was introduced when we moved into TAATS. The original software and procedures were very clunky, especially for OCTA traffic, and we were all unfamiliar with them. The promoting of calls was to give us a chance to find your flight data record or electronic strip or start writing things on the TAATS 'scratchpad" (which looked a lot like a sheet of paper with blank strips drawn on it, because thats what it was). Many software upgrades and years of practice later and it became unnecessary.
It's exactly like our requirement to say "STAR clearance available" before issuing a STAR.