The thing is, it is OS-wide and does not work only for emails. It is not difficult to add to an application and all first party apps that have an equivalent on Apple Watch, iPhone/iPad and Mac work very well. A typical use case is looking at a web page on the phone, putting it on the big screen for whatever reason (for me it’s often to show a video). Or transition seamlessly from whatever I was doing on my phone whilst commuting to the desktop. Or the other way around. Or, having received a message on the watch, if a long reply is needed, the shortcut is there when I get my phone to go directly to the right message. Having that shortcut in a predictable and permanent place means that it’s trivial to do, regardless of the app in use.
It is not huge, but it is the opposite of the thousands paper cuts. It’s a reliable features that provides thousands tiny time savings and some small convenience. It was not uncommon for me to send myself emails with URLs of things to have a look at when at home before this sort of things made switching devices much more convenient (Air Drop also helps immensely).