My guess is he/she tows something like a sailing dinghy. As a dinghy sailor myself I am fortunate that my wife has a Mondeo fitted with towing equipment. If I was on my own I wouldn't run two cars and would have tough choice to make. Swap the Z4 for something "suitable" for towing which I would only do a couple of times a year, or if it can be done fit a towbar to the Z4 and be done with.
I might get scorned by the motoring enthusiasts but the sailing enthusiasts would think its a blast when I turn up with a boat on the back!!!