FileVault is a feature of macOS that offers full-disk encryption for your Mac. This protects your files if someone were to steal your Mac. Without FileVault, someone with possession of your Mac's hard drive could view the data in your files. With FileVault, that data is encrypted and can't be read. It is unlikely that you will need FileVault, but it is still recommended if you are using a portable MacBook that could easily be stolen. Some companies also have policies that force employees to use disk encryption.
Dark Mode in macOS Mojave is a great new feature, but Apple didn't give us a way to quickly switch between light and dark modes. The little donation-ware app NightOwl gives you that control in your menu bar. You can also set a keyboard shortcut, select apps to stay in light mode, and set specific times to switch between modes.