I have a modern heated towel rail in my bathroom, fitted like a radiator into the CH system. Can somebody tell me why it has adjustable valves at both the inlet and outlet points? How does that work?

I would look at the instructions that came with the towel warmer. If you do not have this then you must Google the maufacturer of the towel warmer and download the manual.

What's the brand name of your towel warmer? Does CH stand for heating and cooling?
Steam or hot water heating? Do the valves have markings on them? Is this just a towel warmer,(see image) or is it a combination towel warmer and bathroom heater? (see images) Details! We need details! Regards, tom

Every plumbing fixtures has to have cut off valves so when it needs service, the valves can be closed and fixture removed.

Your towel-holder is part of you hot water system. Its like a flow-through baseboard radiator. One valve controls water coming in and the other water coming out. If you close one valve, water will be coming from the other side. That's why two valves.