'primarily a movement of low achievers trying to cripple the progress of high achievers, or falsely lay claim to the same rewards as them.'

People have different negotiating skills and different level of information about job market.

Knowing how much other people get paid for the same job can be an incentive to renegotiate one's salary or change an employer. Removing part of information asymmetry allows to sell your work for the money closer to its real market price.