It's a long time since I worked at Heathrow but in those days the ATIS was individually recorded at the Sups desk in Approach. Maybe it is automatic now, I don't know.
Assuming it is manually recorded, it takes time to do and the GMC/GMP controller will be in possession of the new QNH, if it has changed, a little while before the ATIS is updated.
There can also be a time gap between the pilot listening to the ATIS and contacting ground.
Reading back the QNH with the ATIS letter would identify to the controller whether or not you have the latest QNH. It fills in the gap between listening to the ATIS and calling GMP/GMC.
As having the correct QNH is a safety issue I don't see the problem in confirming you have it right. Better safe than sorry.