Unions can ensure that the employees get a fair share of the revenue in proportion to the upper mgmt. They also can remove artificial promotion hurdles, that at Google happens according to destructive internal dynamics.

Such dynamics result in deprecating useful and loved products and instead promoting parasitic internal interest groups that can stranglehold growth and innovation in the company.

Ultimately, unions give the employees a voice in shaping the company. Now they can walk out, write posts on internal mailing lists, but at the end of the day they're still in a dictatorship, and they have to accept the decisions of the higher ups, and they won't be asked about their opinion.