They organise a range of events, and represent you to the Guild of Students and to the University. They're all round cool people.
Part of your rent goes to funding what they do, so let them know what you want them to do with it.
They'll promote the facebook groups as well, so you can speak to other residents. Your RA will let you know through there what they are doing in freshers.
Make sure you join the correct group, as some club promoters try to create their own groups posing as "official" to push their unrelated club nights, and you don't want to buy tickets to the wrong events.
The roles include: President, Treasurer, Representation Officer, Events Officer, Volunteering Officer, Sports Officer and Publicity Officer.