Airline marketing teams use the on time performance as a tool to show how much better they are than the opposition, so OTP has always been a key marketing tool as well as an operational efficiency analysis tool.
As BDBM has stated there’s the ‘brakes off’ element too. If you have a significant CTOT delay and can remote park, brakes off and push back is counted as your ‘departure’ time statistically. This counts for both internal airline and external (industry) OTP analytics. Therefore even if you sit for half an hour before taxi, you’ve departed ‘on schedule’. This works even if it’s a remote stand rather than terminal.
Therefore the delay can be attributed to ATC/slot rather than other operating criteria. It also saves having a push back crew held for you, and frees a terminal parking stand for another movement.