An appointment is a contract. It says "I recognize we need to do X together, and I know that X is important to you (or me or both of us, whatever). I promise to be ready to work with you on this at 9 o'clock."
That's right, it's a promise. And we break this promise pretty casually all the time (I'm not very good at being on time either, especially lately).
We make promises all the time at work. Deadlines are promises: "I will have this report to you by next week." Being a manager is a promise: "I promise to focus on making the company successful by making you successful and to try and not waste the next 5 years of your life on useless drivel." Being a part of the company is itself a promise: "I'll give an honest day's work for an honest day's pay."
We are usually very aware of the consequences of breaking these other promises. In only a few missed deadlines we've seriously damaged our reputation and strained the fabric of trust that swaddles us throughout our work and private lives.
Happily—for me at least—we are collectively more forgiving of those that are late.
But this doesn't mean that breaking a time promise is without consequence. Every time you do it you are sending a subtle message to the person you stood up that they are less important than you or whatever you were doing. Do this enough, and you'll strain your ability to work effectively with person for the long term. It also says that you aren't dependable, reliable, or maybe even honest. And let's face it: all that adds up to making you an icky person to work with.
If you are habitually late, decide to fix it. You probably need a few tips, right? Me too. Happily, Penelope over at Brazen Careerist has 5 simple ways to stop breaking these little promises.