This is usually for three reasons: aesthetics, to avoid eye glare at night, and on devices with OLED displays, to extend battery life. The latter is because bright pixels on OLED screens use more power than dark pixels.
WhatsApp even made an amusing video, below, showing people blinding themselves in dark environments with their phone screens:
The Facebook-owned company said:
“Users on Android 10 and iOS 13 can use dark mode by enabling it in system settings. Users on Android 9 and below can go into WhatsApp Settings > Chats > Theme > select ‘Dark’.”
So if you have dark mode enabled on your Android 10 or iOS 13 phone, WhatsApp will go dark with the rest of the phone. It’s good to see you can also turn it on on older Android phones within the app itself when the system-wide dark mode might not be available.
It looks like any iPhones without iOS 13 won’t get dark mode, but Apple currently still updates every phone since 2015’s iPhone 6s with iOS 13, so if you have any iPhone from the last five years then you're all good.