Someone organising an event has the choice between handling ticket sales themselves or contracting it to a third party. Many organisations don't have the resources to have someone on standby to handle ticket sales, and if they can only be sold during someone's spare time they are likely to lose sales. An agency can advertise more widely and and operate in normal working hours, and if they are online provide a 24 hour service. However they are businesses and expect to get paid for the service they provide.
There again, there's always EBay. Set the price at what you think the tickets are worth, and all transaction charges etc. are dealt with via EBay/PayPal. Buyers see the price they'll pay upfront, and this includes all charges, which will reduce the net price paid to the promoter.
Not ideal, but a possible alternative.