The same way any other unionized employees do: higher wages, better benefits, collective bargaining, serious representation in the corporate hierarchy, better job security.
What better benefits could a Google possibly want?
More vacation time, more reasonable working hours, a more transparent process for promotions and raises, better paternity/maternity polices, a seat on the board and more say into how the company is run.
There is a lot more to collective bargaining than salaries.