It's usually a licencing and insurance dodge.
For example, someone with a Gilera Runner 125cc could "kit" it up to 172cc for the extra performance but "forget" to declare the change to the DVLA and their insurer. This means that, to all intents and purposes, they could still ride the scooter on a provisional licence with a CBT.
The problem is that it is actually illegal to do. Not only are they failing to declare a change of vehicle details to the DVLA, but they are now riding a machine for which they are not licenced and will have invalidated their insurance at a stroke. This may never be a problem in everyday usage, but you certainly wouldn't want to get involved in an accident with someone riding under such circumstances.
Such subterfuge may never be discovered by an MOT tech or insurance assessor, but then again you never know. Insurance companies are renowned for wriggling out of claims if they feel they have any grounds for doing so. Sometimes all it takes is the sight of a tuning company's name stamped on a part for a company to refuse to pay out.
