It's an allotment forum. Everyone who has an allotment says that it has widened their circle of friends/acquaintences. So do you talk only about growing fruit/veg when you talk to your neighbouring plot-holders? Of course not. So that is what the shed is for.
If you only want to talk about growing stuff, then why not put the shed, watershed, pets and any other boards you're not interested in on ignore. Easy. I have ponds and wildlife and the competition stuff on ignore - I don't have a pond, am not really interested in wildlife and am uncompetitive.
With a forum of over 9,000 members (although a very small percentage visit or post regularly) there is bound to be a wide range of interests, and A4A does cater for everyone.