I competed in my first meet in over 20 years - which of course was back in the day before bench shirts. I noticed that I lot of the guys with bench shirts were warming up with board presses. Is that so that they don't completely stretch the shirt before the official attempts?
It because lock out work is more important in a shirt. A shirt makes weak points different.
Some warm up with boards because they can't get the warm up weight to their chest with their shirts.
Maybe in training...
It would be to groove the movement pattern and get the shirt set before they take anything to chest. As the shirts advance, the warm-up protocols change. I imagine they've come a long long way since you last meet in the early 90's.