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 it comes to work, I "Reply All" by default for this exact reason. The only time I modify recipients is if I'm starting a side conversation that not everybody needs to be involved in.
Broadcast emails, or emails asking for individual responses are the only time I would use "Reply". However, I think in those circumstances the sender should be using "BCC" rather than "To" or "CC" in order to prevent annoying "Reply All" messages.
If the receiver is an automated mailing list that includes the whole company (or large parts of it), you can reply all with "Hey IT, just checking if this works, because I really have no possible reason where I need to send to this list and the mail server should block it."
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.
This does seem to be the correct reason, at least statistically speaking!
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.
For me, when the email is sent to a specifically created distro list. The whole purpose of the lists is so that everyone with a reason to be in the loop gets updates, even if not everyone knows every single other person on the list.
When there's two people in that list.
