Why do you have memberships rather than tickets?
The Discworld Convention is run by fans, for the fans. Everyone involved in helping to run the event, from arranging a hotel to sitting on a panel or setting up the chairs, is a fan of Discworld.
To buy a ticket implies that you will turn up and that people will entertain you, whereas becoming a member means that you are part of the community: You will still be entertained, but also entertain others; you could join the Facebook group and plan anything from costumes to events (let email@example.com know if you do that!); join in events that others are running; join the Guilds and start whole new activities; get to know people and realise that you might not wake up for breakfast because you've been having too much fun with a bunch of new, friendly people in the bar with a few drinks and a guitar.
Being a member, the possibilities are endless1.
1 For finite values of endless. Please consult Hex for specifics.