Gotta remember that just because you put something in a contract doesnt mean its legal. I could put that you must murder anyone i tell you, even if you sign that it doesnt make it legal. But thats criminal law.
I terms of the UK, realistically its still just a curtousy, but employers put it in because employees often thinnk that they have to adhere to it, they don't. If in the scenario the employee doesnt work the notice period and just immediately leaves, the employer technically could take them to civil court, make a case that they need them to finish their notice period, and the court could theoretically grant them that due to the employee's breach of contract. But i'd imagine thats almost never happened (just guessing), because that would take considerable time, effort and money to do that, and its less hassle just to let them go.
So yeah, its put in because employers can, and employees often incorrectly think they're beholden to it and comply anyway.
If all the companies have to wait it just becomes normal.
It might not the best advice in Germany, where a reference letter from your previous employer can be very important for a new job (maybe not so much for expats).
I terms of the UK, realistically its still just a curtousy, but employers put it in because employees often thinnk that they have to adhere to it, they don't. If in the scenario the employee doesnt work the notice period and just immediately leaves, the employer technically could take them to civil court, make a case that they need them to finish their notice period, and the court could theoretically grant them that due to the employee's breach of contract. But i'd imagine thats almost never happened (just guessing), because that would take considerable time, effort and money to do that, and its less hassle just to let them go.
So yeah, its put in because employers can, and employees often incorrectly think they're beholden to it and comply anyway.