Good managers can be a rare breed, so it is important to count your blessings when you experience one. While it may be unwise to think of them as your friend, they make an employee's working life significantly easier if they are doing their job properly. They are the type who you don't always fully appreciate at the time, but if somebody bad replaces them, you can bet that you will feel the difference.
Just because a boss is competent in their role doesn't mean that they will stick around forever, whether that is because they have decided to transition or the company has decided that they are no longer needed. As an employee, it is important to be ready for this change, which can be destabilizing. You never know what might come next at work, and it might even get to a point where you are expected to fill their role. While this can sometimes be an honor and a welcome next step, there are times when this may be more of a dilemma than you want it to be.