[Anderkent]

This assumes that leadership has perfect knowledge of the situation, which is just never the case. Unions can be an additional source of information about the state of the company, for things that are not being communicated via the usual management structure.

[acdha]

That last point is key: a union exists outside the management hierarchy. There are countless examples of situations which are well known but ignored for political reasons because everyone involved reports to someone with a vested interest in the status quo. A union can be extremely useful for forcing things into the open and doing so in a context where people feel safer commenting because they’re not the only one drawing attention.