Dark mode is especially useful for devices with OLED displays, as well as being easy to the eyes when reading in dark environments (i.e. in a dark room). And with this beta release, WhatsApp is the third Facebook app to include dark mode - the other two being Messenger and Instagram.
However. WhatsApp Beta is only available for select users who opted-in to join the beta testing, and there is no announced date for its public release yet.