You guessed right. Works as as a spacer and keeps the bullets from toppling when moved. Once it is in the oven, loading bottom to top, I just gently lift it up and off. Then do another tray.
It rained every day here last week in the Gulf Coast, so I took advantage of being indoors and cast more than 15k of bullets. When I say indoors I cast in my garage with the door open sitting in front of a 48" shop fan, which vents to the outside.
The problem is our humidity is 100%, so per-heating is mandatory for PC. One of the benefits of a per-heat at the upper limit is a coating that is not fragile like powder held only by static. Setting the bullets in the grid dosen't damage the coating. I do a very high per-heat (300 degrees) when PC hollow points as I spray base up so I can keep the powder out of the HP cavity.