From time to time, someone will raise the question, "Is courtesy dead?"
As long as humans exist and continue to interact, we will need courtesy. It's the social lubricant that allows us to pretend that what is happening isn't happening so that we can get what we want and need.
Take a step back.
At one point, we all thought, with good reason, that the world revolved around us. We expressed a need or desire - food, a diaper change, a toy - and it was quickly given to us.
After awhile, though, and this happens tragically soon for some, we discovered that the hands that provided for us were powerful and could withhold the wanted or needed item or event.
Even worse, those hands could be agents of anger and inflict pain.
After that, most of us realized that we weren't guaranteed what we wanted or needed. Whatever it was - a home, mate, livelihood, etc - we had to do something to get it.
Those dynamics are constant. Everything that you do - everything - is meant to either get you something that you want, or to get you out of something that you don't want.
How do interact with others to get what you want and need?
Part of the answer is courtesy.
When your boss comes to you with an assignment, s/he will probably "ask" you to do it. That's courtesy, covering up this reality: unless you have a very good reason, you complete the assignment or you risk losing your job.
Courtesy makes it palatable, though. The job gets done and you get to preserve your sense of independence.
It works the other way, too. When you approach with a request to use vacation time, you frame it as a request.
That time belongs to you. Unless the company has a dire need for your services at that precise moment, what you ask for is more like notice that you won't come to work on those days.
The courtesy of the request allows you to make a demand of your boss without seeming to be aggressive.
Courtesy keeps us out of fights that we cannot win by smoothing over the various differences in power and status.
Does a man really have to hold a door for a woman? Of course not. The gesture is a signal that he's going to use the advantage in strength given to him by biology in a benevolent way.
If you ever happen to meet a monarch then you'll bow, a gesture that recognizes that person's status.
Is a king truly superior to you? Probably not, but he has the power to enforce his perception of superiority by harming you, so you'll bow, play along, signal your non-challenge, interact in a way that gets you want you want.