If you want it. Not for me.
Use cases and aesthetic subjectivity has to be taken as read, here.
That said, dark-theme showed up quite a long time ago as the default in some Linux applications in certain distros, and I (with full-throated aesthetic subjectivity) loathe it. Glad it works so well for some people. Some people love the live-tile stuff in Win 10, too, but to me it's like having brightly-colored cockroaches on my screen, squirming uncontrollably.
However, my involvement with MS Outlook will last, at most, three more days. [cue old-guy laugh] Heh, heh, heh. Beer, because I'm having a retirement party.