Why do they need it? Besides dumb envy, why would I need someone's tax return? What's in it for me?

Salary negotiations are a very simple example, you can easily compare your salary to that of your peers and to similar positions in other companies. If your boss tells you they pay you the industry average or company average or whatnot, you wouldn't be able to check whether that's actually true otherwise. You can also have a rough ballpark of what a company pays before you apply for a job there. In general, information like this being public empowers people, whereas in most countries companies hold all the cards and use this information asymmetry to their advantage.

Fairness and efficiency. If someone is making significantly more money than you, they are either:

a) creating more productive value than you or doing something more in demand by society [strong signal you should join them!]

or

b) manipulating their situation for better outcomes unfairly or fraudulently

In both cases it's in the interest of the greater good to have these things out in the open.