There are several sides to this story
Firstly every Airline has to choose a route to market; either direct sell to brokers and forwarders and sometimes end user or they appoint a GSA. The benefits of this are that a GSA isn't a fixed cost and all ready has a office network and client base.
Knowing how unreliable Cargo airlines, a lot choose the GSA route as one customer is easier to deal with than 60 plus all wanting to know if they've shipped.
One of the key things is a good broker can match add-hoc with timetable and the right equipment with the customer.
A good sales department would cost £100,000 minimum possibly £250K which is a lot to fund running 3 DC 10's