Each local party has people elected to their Executive Committee as Ordinary Members, who do not hold specific roles. These are people who help by having a say in how the local party is run, and by taking on ad hoc roles and helping with special projects.
Being a committee member enables you to help guide the party in the best possible direction, and you can do it without taking on a specific functional role. It suits someone who is generally quite competent, and who likes to look at the bigger picture of their local party. It's ideal for somebody who wants to be involved, but who may not want an official position.