In theory the unions should be a place to organise employees, to help improve working conditions. But I suspect for most people the real benefit will be protection from miscarriages of justice.

I've personally seen people put through the wringer by HR teams, and even when the HR team acknowledges they've fucked up, there's no apologies or an attempt to make things right. Unions can provide protection in these case, whether that's access to legal help, or just having a 3rd party on your side sitting in on employee dispute meetings.

I think a lot of people don't appreciate how badly they can be screwed over by a HR team, accidentally or maliciously, until they find themselves in a meeting with three members of the HR team, with no one telling them what's going on. At which point, it's already too late to save yourself. A union gives you recourse and support, something invaluable when it you're up against the entire HR team.