I never understood it but it puts a smile on my face everytime I see it.
It's simple. It's called being a...guy. We like mechanical things, things made of steel (watch Conan the Barbarian), we like gigantic front end loaders...and we like knockers!
I don't know what I'd use that shackle for...but, now that I think of it, that would make a wicked door knocker.