Especially important if you do a lot of design work
Installing fonts without any organization eventually clutters up the font lists inside your other apps and can slow them down, so it's worth building a habit of keeping only the fonts you actually need active and deactivating the rest -- a small habit that pays off in day-to-day efficiency.
A quick way to spot font conflicts
Font Book also flags duplicate fonts automatically and lets you resolve them right from its own interface, so if a document's formatting looks slightly off across different machines, checking for duplicate or conflicting font versions there is a good first step.
Frequently Asked Questions
Does a deactivated font still take up disk space?
Yes -- deactivating a font only hides it from your font lists and apps, it doesn't delete the underlying file, so it still takes up the same amount of disk space.
Will Font Book tell me if I have duplicate fonts installed?
Yes, Font Book automatically flags duplicate fonts and gives you an option to resolve them by keeping only the copy you want and removing the unnecessary duplicate.