a British phenomenon in the motorcycle world (thanks Tony!)

 If you were a learner motorcyclist on a solo machine you were restricted to 250cc (which meant a top speed of around 65mph) and you couldn't carry a passenger, not much fun if you wanted to take your girlfriend somewhere.

But if you rode a sidecar outfit, neither of these restrictions applied, so you could get hold of an old BSA 650 or similar, pull off the humongous large double adult sidecar and restore your street cred (and a fair bit of performance) with something small and light, such as a wooden box, a bath tub, an oil drum, a coffin if you were an exhibitionist, or even just a plank with a seat bolted to it. 

You could now take your girlfriend everywhere, at least until she found a nice boy who could borrow his dad's Mini. You also became very popular with your mates, not only could you give them lifts when their bikes had broken down again, but when you went on group runs they could all dump their luggage on you. This situation lasted until around the early 80s.