Those union leaders are hopefully elected by employees (otherwise, how do they get to be leaders?) so you can align the union leadership with your interests by voting for someone whom you trust to represent you. Of course you don't need that if management is also elected, but I don't know of any company that does that.
It's not about whether I trust the leaders. Even if they're trustworthy, their job is to work for what's best for the union as a whole, and that's often going to be different than what's best for me. As anyone who's worked in big companies can attest, there are all sorts of ways that facially universal policies can have disproportionate impact on specific people or teams.
