What is the importance of Thanking someone?

Good manners are always a must out in society. People do notice! Sometimes it's important people and could mean you getting a promotion or a good job because of good manners and how you handle people.

One generally says 'thank you' when someone does something for you that they didn't need to do or they are simply helping you out. Reverse it .... what if you did something for someone and they simply walked away without a backwards glance. I doubt you would like it.