1) When workers want systemic reform of an organization.

2) When workers are more-or-less interchangeable.

Software engineers aren't interchangeable at all compared to a factory worker, which is why they have such great individual bargaining power and why (historically) they don't really feel a huge need for a union to represent them. Today, software workers have a bit more interest in unionizing because they want to change their organization's high-level goals (e.g. environmental responsibility, not making tools for warfare, hiring diversity, etc).

But both scenarios apply to Amazon warehouse workers, and union representation would almost certainly be a massive benefit for them.