It only influences how long a player can be trained, but there's a lot of variance regarding the point where a player doesn't receive optimal training anymore. Very very roughly, just to make it clear, you could see it as:
All star: 50k salary
P. All star: 75k salary
Superstar: 100k salary
MVP: 150k salary
HoF: 200k salary
ATG: 200k+ salary
This is in no way accurate let me emphasize that, but after your player reaches that salary there's a good chance his training goes a lot slower. Even if he reached his cap, he will still gain a little training.
The point where a player caps varies because of the skill distribution; if you train a player only in his primary skills he might get bigger salary, but his overall skill point count will be lower than a player trained more balanced. The more balanced player might cap sooner, but will have more skill points for his cap.