I see the workplace benefits of a global union (specifically the IWW) as a bonus, with the real meat being that it teaches people how to organize, and how much power they truly can wield collectively, as many people still feel quite powerless despite the potential they hold, they need only be taught how to use it.

When the Spanish Civil War kicked off, it was the Syndicalist unions (CNT-FAI) that were able to organize their communities effectively to resist Franco and transform Catalonia when the existing government crumbled. That type of organization doesn't necessarily have to be from a union, but I feel the ability to engage in a general strike would be far more encouraged if people were in a union and became used to flexing that muscle (and build up a strike fund to be able to sustain it) and would drastically help in effectively resisting.