How might a unionized shop benefit Costco workers?

Let's look at history:

"Costco Wholesale Corp and union negotiators have reached a tentative agreement on a new labor contract for 13,000 Costco workers in California, the International Brotherhood of Teamsters said. The agreement raises annual wages, lifts semiannual bonuses and increases retirement packages."

These are heartening results, but what I like about a union more is that it is a vehicle whereby the hourly part-timers at Costco have a voice to challenge the company's confounding policies about scheduling, seniority and task allocation.

As things stand now, we have no voice.