In theory they are a body representing the interests of small independant companies. They share expertise and knowledge amongst members and act as a lobbying group towards local and central government policies.
For example if the council were to propose businesses had to pay a new fee for some sort of service it would (in theory) carry more gravitas if opposition to that policy came from the chamber (representing say 200 businesses) rather than some or all of them obecting singularly.
Bit like the taxi association?
As far as I know they are funded from members subscriptions and income from selling services.