My daughter is working at her third consecutive Glasto, having failed to get tickets each time. My understanding is that the event is 8-10 times oversubscribed, and if just a small percentage of those unsuccessful applicants are prepared to work in exchange for a ticket, then the event is cheaper for everybody. She works for a charity ( firstly Wateraid and now Oxfam ) , who presumably get paid for providing staff ( toilet cleaner first time, now marshal/steward ).
In return for a number of hours work ( 4 x 6 hour shifts, I think ) she gets full festival ticket, exclusive camping area with showers, access to staff bars and food for each work shift. If they fail to complete the work aassigned to them, then they have to pay the full price for everything provided, and will never get invited again. Almost never happens. Apparently, there are many people who do this every year, and the charities tend to look favourably on applications from previous workers. Usually, they are expected to do another festival too, to get into Glastonbury.
This year, she's been invited to go early ( she went down on the previous Saturday ) and do all her shifts before it all kicks off on Friday. She was well chuffed about that, but it means spending 9 nights in a tent !