What is “reply-to-all” behavior?

Reply to all sends your response to the address in Reply To if present or From otherwise and also to every visible recipient in the To and Cc fields. Hidden Bcc recipients do not appear and are never exposed.

It is the equivalent of standing on deck and speaking to the entire crew rather than whispering to one officer. One click can grow a thread into a full crew conversation, which is useful for collaboration and risky when the wrong people are included.