The wiper mechanism as the term suggests is a series of gears, metal rods and motors which help move the windscreen wiper back and forth across your screen. The wiper mechanism tends to become rusty after a while in which case it needs to be replaced. However, even though all wiper systems use the same principle the actual construction, size and weight of the mechanism vary depending on the vehicle you own. It is for this reason that the mechanism is not interchangeable.
In the vast majority of cases, the windscreen wiper mechanism does not need to be changed. Sometimes it's only the motor which is at fault and can be swapped out for a new one. However, in the event of an accident or if the mechanism has worn out will you need to buy genuine wiper mechanism spares. The most significant sign of a worn out wiper is significantly increased sound from the mechanism when the wipers are turned on. Also, one of the wipers may fail to function as it should or may not be able to wipe the screen using a full range of motion as it did. If you notice any of these issues, it's best that you get your car checked by a competent mechanic.
Even though most people don't think twice about their windscreen wiper mechanism it is still a very important part of your vehicle especially in the UK because of the prolonged wet weather. It is for this reason that you should only buy branded replacement parts like from brands like Magneti Marelli. That way you can be assured of years of service life.