In your case, the fees for the JFK-GIG would be $ 5 + avios and for the GIG-MIA $ 54 + avios, so if the whole itinerary is on one ticket and you cancel it, you lose $ 55. If it is in two tickets and you cancel both, you lose $ 59, which is a very small difference and probably makes it worth it for the extra flexibility.

(Edit: I see that you have posted pretty much the same message while I was writing this one.)