It's for parking during the day before the match.Most car parks have an evening rate so unless you are arriving particularly early then you should be able to park.
The Trafalgar Street car park - just a couple of minutes walk from the station - is £6 if you arrive after 6pm and leave before midnight.
If you are arriving after 5pm, you can get the night rate at the Theatre Car Park on Church Street for £7.95 or save a quid if you use the App. From there it is about a 10 minute walk to the station or down to the Old Steine to pick up a bus to the Amex.