Why Fixed/SS hub instead of Fixed / Fixed?

I bought a fixed/free hub first time round for the simple reason that a) I wanted the option to use a freewheel and b) I had no idea you could screw a freewheel onto a fixed hub.

I suspect that most people do not realise that you can run a freewheel on a fixed/fixed.

I didn't know that, for example. But when you think about it, it makes sense.