When you choose "Book now & pay later" you are reserving the space so that no one else can book it. You have to make a payment within a set period otherwise your booking will be cancelled.
Each venue can set what this period is, and it will be displayed when you checkout under Payment Options.
Once you have made your payment, you will receive an email with a summary of the event booking, and an invoice that you need to pay by the due date. There are no costs if you choose to not pay within this period, although this will be recorded against your account and displayed as a ‘reliability score’ to the next venue you make a booking for.