I very often have at least one set of custom fields which are tied to the home page only. Unfortunately, these are tied to the front page’s IDs, and it’s possible that down the line a client could change which page is the front, for whatever bizarre reason they might have. This would remove the fields from the admin area.
To make it more reliable, I usually create a new variable representing get_option( 'page_on_front' ); and set the location rules to it. But this will obviously not work with the ACF-JSON files which have been implemented in v5.
To get around this, I propose a location keyword which represents whichever page is currently set as ‘page_on_front’. This will allow maximum portability whilst still allowing use of JSON