European here. Having 3 months written in your contract (and in many countries by law, like France) is pretty common.

You can still quit if both parties agree. I've done it before.

The clause is there to avoid this type of scenario where a key employee just says fuck it and leave.

It works both ways. So a company can say fuck it and fire you just because they want to.

It's a fantastic system. It can be exploited, but usually works really well for employees and it gives you a huge safety net you can (usually) rely on.