I’m not defending top posting here. I’ve noticed a couple of practices in companies (not sure about other environments). One is doing a Reply All, adding a reply body and copying some new people into that thread after several back and forth messages have already been exchanged. Those new people (can) now have the full context of what they’ve received without the person composing the reply having to add context. The second is when people send mails to several others and miss copying some. Then one of the receivers or the sender themselves would do a Reply All and add a “+<person forgotten before>” (or sometimes even use ++ and the name).
