Ask HN: Why salaries are not openly discussed?

In some of the companies I worked for (or know of) disclosing your salary was not allowed by contract.

But in most — there was no serious barriers, but still it was never talked about among colleagues except between very close friends.

Why is that, i wonder?

Why it is sometimes in the contract — what does employer gain from it?

Why people tend to avoid these discussions even when it is not forbidden?