They offer you a £3 reduction if you buy in advance. You will attend if you purchase a ticket.
If they have the same “pay on the day” price no-one will buy in advance. Then, if the weather is crap, you got hammered on Fri night, Scunny got hammered on the Wed night (very possible), or the missus wants you to go shopping, you won’t bother going.
Which is all very well until the weather turns out to be nice and people think “might go to the football” only to decide “£21? No chance”.
Of course it could be seen as a plan to encourage people to buy season tickets. I’m not sure of how successful that might prove, we’ll see when the season starts.
If that were the case, it would have made sense to publish these prices during the “early bird” season ticket period, surely? Then you could see what you were actually saving.