There's more to it than that. Firstly we can give someone 1/19th of their seasoncard cost back and sell the seats at full price. Most of the seats unsold yesterday will have been single seats, which are no use to someone wanting to take their kid or kids to a game maybe for the first time. Those people could well be our future seasoncard holders.
I started by nagging my dad, who took us to a few games to start with in the 1960's, then we got season tickets in the early 70's. Going to those few games made us fans for life. Not having seats available for sale, just because people can't be arsed listing them, stops these people attending.And it's not just seats they but but merchandise and refreshments.