The benefits of joining
By grouping members into a geographically defined site the public don't have to look all over the place to find you.
When they look at an area site they can see a range of activities which will give them a reason to book mark the site. As they come back for further information they will be drawn to look at other sites which are promoted.
The Site reflects much more than businesses, it takes in the whole feel of the area with information about local events, charities and culture so that browsers can build up a relationship with the site and may even be contributing to it.