presumably
it's a prepay to secure the spot and product.
That cuts down on lines and waiting etc.
If it is available on an app and at the con, is it still only available at the con? then you prepay, skip the lines, and still have a comic con experience that didn't take too much time and etc still guaranteeing a product
.... also you can use a card at home before the con, most people use cash at the con, to avoid card scammers.
Perhaps even prepay so far in advance that it doesn't take away your cash amount for the con.
Especially if it is only available at the con.
Then they also do it for guaranteed spots sometimes, and then there are also non guaranteed spots that make a line and etc.