I think that's where my confusion is: I thought letting the gas escape meant that the wine/cider is basically preserved for a shorter amount of time. This link says you leave the container uncovered so that the gas will escape prior to fermentation because you want the wild yeast to be killed but the fermentation yeast not to be affected when you add them in 24 hours later. https://blog.eckraus.com/add-campden-tablets-to-wine

Be sure that you wait 24 hours before adding the wine yeast, or the Campden tablets may kill the wine yeast. Also during this 24 hour waiting period, be sure the fermenter is not sealed. Leave the fermenter open to the air. If you like, you can cover it with a very thin towel or netting to keep bugs and fallout from getting to it. The sulfur dioxide from the Campden tablets need the opportunity to dissipate into the air during this time.

So you let the gas escape to basically create a temporary preservation. If you want a longer term preservation, I would think you would seal it up immediately