It helps you to make roster decisions. You set a scheme to fit your style of play, and scheme helps you identify players that fit your style of play.
I disagree. Why should scheme adjust player ratings? In theory it would affect production because players would fit your style of play and therefor perform better. But that is if you know what your scheme is and how to benefit from it. If you sign a bunch of cover 2 corners and they try to play man defense that is on you.
That may not be a problem with scheme as much as it could be a problem with ratings in general. He may be rated a 65 ovr, but what is his speed? What is your team scheme and what RB type do you have set? What kind of plays do you call? If your RB type is set to power and you like to run outside zone stretch plays then the problem falls on you. And again, I am fairly certain that scheme does not affect player salary. It may cause a team to overpay someone, but a player will not take peanuts if his base overall is 85 but your scheme rates him as a 65. He only cares about scheme if it results in you overvaluing him.