There may be many use-case scenarios for the JSON field introduced in Frappe14- Beta 4. I request community members to share their opinion and experiences. I understand one to maintain masters type doc types if these masters needed to be added and maintained from the Admin side only.
Might be applicable for child records where there is a one to many relationship (child within child).
For example, an item line may have:
batches (json field)
and that 10 qty, may have 3 batches:
so the batches json field in the child table can contain…
this is not how it is implemented but just an idea that one can store one-to-many information in json fields