You as the employee with that skill DETERMINE the market value. If every IOS developer refuses to work for $40k and want $80k the market will start having to pay $80k. Yeah there will be outliers, maybe someone desperate for a chance to prove themselves, or someone who was unemployed will take that $40k but that is not the market value.
It's not some vooodoo a company makes up, you yourself have a direct affect on the market value by turning down and taking positions. If I can't find anybody to hire at $40k I might start offering $60k and more until I get the amount and quality of applicants I desire.
Right, but it'd be easier, for example, for every iOS developer to not take $40k if they knew that other developers are being paid more.
Basically, keeping information private gives an advantage to the people who know the private info versus people who don't. If no information is private, that edge is lost, aka, a more level playing field.
It's not some vooodoo a company makes up, you yourself have a direct affect on the market value by turning down and taking positions. If I can't find anybody to hire at $40k I might start offering $60k and more until I get the amount and quality of applicants I desire.