Here's a use case. I have a family sharing a login (CRM record) and I want to store some custom data against that CRM record.
Best way is via Custom Field sets, I can define the field types, I can import into them, they are great!
But they aren't currently editable via the UI - I can only change the data by submitting a form or via import.
Custom Fields sets could be better if:
a) They were editable
b) (nice to have) I could add a custom field set to a CRM record without filling in a form