For the most part the update to the iOS and macOS app doesn’t change how the app functions or how it is used, save with the option for users to enable/disable dark mode. If you use your computer or phone in low-light situations like in bed at night, then dark mode might come in handy if you’d rather not go blind from the glare.
That being said, Evernote points out that there are still some areas of the app that aren’t dark, but state that they are working to fix that. “We were excited to give you dark mode as soon as possible, even though there are a handful of areas in the app where that’s not the case yet. We’re working on fixing that. In addition, some items—like images you’ve added with white backgrounds—may retain their white backgrounds even when using dark mode.”
As for Windows or Evernote Web users, the company mentions that they do not have a timeline as to when dark mode will arrive for those platforms yet, and are focusing their efforts on improving performance before introducing the option.