In the US, mostly worries about social problems from disparities. Parents may not want their kids to know if their family is substantially wealthier or poorer than their friends' families. Heck, adults might not want to know it either. At some level of very-wealthy or very-poor it becomes obvious, but there's a broad set of salaries in the middle that are broadly lumped in as "middle class". Many people in that tier think of themselves as just middle class, and socioeconomically roughly on par with other middle-class people. To maintain that polite fiction, it can help not to know that you actually make 2x as much as your friend, or vice versa (in many middle-class communities, salaries of both $120k and $60k can be found).

Companies similarly worry about a hit to team cohesion that could result if team-members find out that some of them are paid much more than others.