In short, while it’s easy to get too worried about what your salary says about you as an applicant, it’s a good indicator of how well you’ve performed on the job, and of how valuable you are to your employer. It’s one thing to get a promotion or two, but, at the end of the day, did your employer “show you the money?”
To get a reliable read on how well you’ve performed on the job, MBA admissions officers only have several things to go by: What you say in your essays/resume/interview, what your recommendation writers say, and your salary information. Obviously, you will say you were great, and you will probably only request a letter of recommendation from someone who will say the same thing. This is where your salary information comes in. It provides a sort of indisputable “fact check” to see how much the company really valued you.