When your message is pertinent to every single person on the reply all.

I reply all to all work related emails because people will add someone who needs to be aware of what's happening and I may not realize this is important to them. If they don't want to be on the email they can ask me personally and I'll take them off the chain.

I never reply all (or at all) to company update posts (e.g. new hires/promotions/other bs). If you want to congratulate them do it privately. The whole company doesn't care.

When everyone you're replying to is a scammer that you added to that list to purposefully annoy them.

When you want to unsubscribe from one of your company's mailing lists but you don't know how.

When you want receipts to prove you did your due diligence without relying on IT to dig it out of the other party's email history should a conflict arise.

For conversations which include interested parts as recipients, it's good to reply to all to ensure they get all the updates. But this goes for smaller groups. If you go beyond 7-10 recipients, it might be time to ask if everyone really wants this or if some other communication format would be preferrable (chat group or meeting).

When others might be interested in what you have to say. If it's a discussion, them other participants almost always want to receive what you send. However, if you asking a question about an assignment, don't, no one else needs that.

When there's two people in that list.