A feeling of belonging is a natrual human charicteristic. People like to mix with others who share an interest, and maybe broader views and opinions. Joining an actuial club or society is a badge of 'membership' in its truest sense - of belonging to a social group, and people dervie pleasure from that feeling of belonging over and above the activity they share. They are usually happy to contribute towards costs for equipment, premises, and similar overheads - it increases the sharing atmosphere.