static files dirs
So say you have a frontend that doesn't run on conventional html and you have static files referenced there with, you might not be able to use Django's static template tag to point Django to those static files but you still want Django to collect them so that you can make them available during deployment. This is the best way to point Django to them without the help of the static template tag.
This is just one use case of the STATICFILES_DIRS settings, there are of course others.