I am a bit biased after your comment. Microsoft Edge is going to have Blink as an underlying rendering engine, the same as in Chromium, Chrome, Opera, Vivaldi etc. etc. You have pointed out that it might "help web developers", then could you explain me how there might be some difference (even negligible) in rendering between those browsers since they all run on the same technology? It's not Trident nor EdgeHTML anymore, there won't be any star-slash-backslash, -ms prefixes, filter() nor other hacks [0] that we had to use for IE. It does not look like Microsoft is going to fork Blink anytime soon, because it will be quite the same thing as they would stay and continue development on EdgeHTML engine.
I don't see any advantage of having Edge on macOS, FreeBSD, Linux, etc. for web developers. However, in case of end-users the cross-platform browser have more sense, especially when they got used to specific features and ecosystem. Lack of macOS browser might lead to churn or a problem with adoption to it.