While it has the added benefit of making sure the vent port is clean and clear, the primary reason it to make sure that all of the air is removed from the canner and that only steam remains.

If air remains trapped in the canner then both the pressure and temp are affected (reduced) even tho it won't register as such on the gauge. It is a food safety oriented rule. ;)

Dave

Thank YOU!!

That's right. You want to measure the steam pressure, not the air inside. Otherwise it will be underprocessed and your food will be unsafe.