Power
It can be daunting. And/or fleeting. If you head a big powerful union where there are no alternatives, The Man (the executives who contract with the union for the services of its workers) quivers when you walk by.
What if you run the loading dock union? Who else is strong or dumb enough to stand beneath huge, heavy, rotting payloads, which must be refrigerated, to load them onto the platforms of Teamster trucks to be hauled away to the Safeways and Krogers of the world? If you ever stopped work, all of that commerce would die and the global economy would find itself crippled. That's the big, fat power side of running a union.
The other side can be ugly. Famous cases all over the land of Wal-Mart where the company came into a small, financially-strapped town with the promise of tons of jobs for the locals. New shopping center. Lots of business activity as shoppers from fifty miles all around are drawn in.
Then a yokel decries that Wal-Mart isn't paying enough to its workers. The yokel gets the would-be workers to form a "sit-down strike"—that is, they just…sit down. And do nothing. A week later, Wal-Mart announces that it's not going to open its store; it'll leave the area and instead move fifteen miles down the road to another cash-strapped small town that really does need the jobs.
In the meantime, the original town of Wal-Mart's choosing goes fully bankrupt. The yokel who called the strike in the first place is tarred and feathered and is now a bank teller in Nairobi.
And there's a third side to the power—technology. Cashiers have threatened for a long time to call strikes. Finally, the various retail stores under their own pressures from Amazon and other e-commerce players explained that they can't survive as businesses paying cashiers $38 an hour. So they created what are basically ATMs for checkout purposes.
They only account for about two percent of purchases today but are a shot across the bow of the boat of the cashier's union, telling them they'd better play nice.