When we attend events and locations (such as a museum), we collect fees in advance to speed up check-in and not make everyone wait in line. We wanna get started shooting as quickly as possible, not stand in line at a cash register. I can't pay on your behalf if you don't pay in advance.
In some situations, paying in advance as a group means we get discounts.
In the past, the majority of people that promised to "pay when I get there" never actually show up. This means the organizer has to pay the venue for no-show tickets promised to the venue (out of the group's budget).