One way to think about it is in terms of economic theory. Negotiations are fairest when both sides have equal power. But you have approximately one job, and a company typically has way more employees. It's more painful to lose a job than to have 1 employee quit. So the negotiation is fundamentally unfair.

So when it comes time to negotiate your annual raise, it's not you alone dueling with your boss (and the whole corporate hierarchy backing him up). Instead it's you and all your colleagues getting together, sending in some representatives, and saying, "Profits are way up this year, so we all deserve decent raises. Let's talk numbers."

(This is less interesting to tech workers, in that we already have a ton of salary negotiating power due to supply/demand differences. But personally I'd still like to see more collective action to deal with other issues. A good example is the way employees recently forced Google to get out of military contracting. Or imagine how different Uber might have treated employees if they'd had a union that helped keep awful managers in check.)