In the case of a Macbook Air or Macbook Pro Retina with ‘limited’ storage on the SSD, this distinction becomes more important in that in an ever rapidly increasing file-size world, you keep vital large media files, pics, video, PDF collections, music off your SSD and archived on external storage, for sake of the necessary room for your system to have free space to operate, store future applications and general workspace. You should never be put in the position of considering “deleting things” on your macbook SSD in order to ‘make space’.
Professionals who create and import very large amounts of data have almost no change in the available space on their computers internal HD because they are constantly archiving data to arrays of external or networked HD.
Or in the case of the consumer this means you keep folders for large imported or created data and you ritually offload and archive this data for safekeeping, not only to safeguard the data in case your macbook has a HD crash, or gets stolen, but importantly in keeping the ‘breathing room’ open for your computer to operate, expand, create files, add applications, for your APPS to create temp files, and for general operation.
Your HD or SSD is not a "storage place" for lots of big data.......vids, pics, audio. etc. 😊