But beyond that, let's say you knew that a certain bank had a good chance of shutting down its branch offices in the town where you live, for whatever legitimate reason. Would you consider opening an account at that bank? Well, clearly no, because those bank branch offices might not be there for you to use in the future.
Now let's say you were a manager and you knew one of your team members was looking for a new job. Would you feel it to be worthwhile to assign important project tasks to that person if you knew there was a good chance they would be unavailable in the very near future and you'd have to move the tasks to someone else? Are you going to put them up for a promotion or a bonus to reward them, knowing that the effort might (in a sense) be wasted? The knowledge that an employee is considering leaving, no matter how much the manager might wish them well personally, changes the relationship because of the uncertainty about the future availability of their time and accumulated knowledge. It's better either for the employee not to create that uncertainty and/or resolve it as soon as possible.