What do we actually gain by unionizing?

I'm not against unions, and I lean more towards them than neutral, but help me understand what would actually happen if we try to unionize?

From how i understand things, we have to get enough signatures from employees in our building in support of the union.

But doesn't the site management have to approve our demands?

If that's the case, then what's the point if they can just basically say "no" or leave us in indefinite indecision.

If we are successful, are we able to get stuff like pensions? Are we able to negotiate a raise?

From what i understand, unions can be site specific. But corporate pays us, so how would a raise work?

I understand what a union could do, but i don't understand how it actually gets done.