I will answer it from an economic perspective...it will give you a new insight...I have taken this example from a book written by Ben Stein
Say you open a book store. You hire J as your employee. During his first year, J is very clumsy and makes a lot of mistakes. Despite that, you are very patient with all his mistakes and you slowly train him. During his second year, he has become more experienced and makes less mistakes. During his third year, he becomes so good that he can even handle managerial stuff. In other words, he becomes more valuable. In fact, your competitor across the street has his eyes set on J. J has 2 options. Stick to your bookstore or move to the book store across the street where he will receive a fatter pay check.
Now, we only have 24 hours a day. You can earn a slow wage increment working at your old place or boost your income by getting a job elsewhere. Which would be your choice? It is obvious, isnt it? The more valuable you are to a company, the more choices you have.
I have a friend in dining services who keeps 'trading up' his hourly wage. Started at $x an hour..then he learns some additional skills in the dining office and he moves on to other dining halls to hold a managerial position..In his new position, he earns ($x + 4) per hour...see what I mean!
There is always an incentive for a good employee to seek employment elsewhere where he will get paid top dollar for his services