When employees reveal their salaries to employers, employers use that as a reason to pay them less. For instance, if someone is moving up from retail to an office job and the office boss sees the retail salary before making an offer, the office boss will offer the potential employee less money. The boss thinks the employee will accept less because of what the employer is used to.

For this reason, many states ban bosses from asking about salary history: https://www.paycor.com/resource-center/articles/states-with-salary-history-bans/