How to Install Fonts Manually in macOS Sierra
To install a new font manually in macOS Sierra, drag its icon into one of the two Fonts folders that you have access to. Why might you want to install them manually? If you install a font via the double-click-and-use-Font Book method, the font will be installed in your Home/Library/Fonts folder, and available only to you.
If you want other users to be able to access the new font, drag the font’s icon to the Fonts subfolder inside the Library folder, which is at the root level of your hard drive. This Fonts folder offers universal access. Or, if you usually want to install fonts for everyone, change the Default Install Location in Font Book Preferences (Font Book →Preferences or ⌘+, [comma]).
The Fonts folder in the Library inside the System folder is reserved for macOS and can’t be modified easily. If you try to remove a font from it — or add one, for that matter — you first have to authenticate yourself as an administrator. Do yourself a favor and never remove fonts from /System/Library/Fonts. You can really screw up your Sierra operating system if you remove the wrong font, so don’t go messing with the fonts in folders unless you know what you’re doing.
Your best bet is to always use Font Book, which prevents you from doing anything bad to fonts.
One last thing: Try to refrain from installing more fonts than you’ll use. Having tons of fonts installed can slow down some apps and make Font menus long and unmanageable. Install only the fonts you need — your Mac will thank you for it.