Locking Fitness Signature

What are the upside or why would I want to lock in an activity fitness signature? Does it relate to the fact that once winter is here I will not be performing maximum peak power effort on my indoor trainer for many months in a row?

Sometimes your fitness on a particular date is known and different from what the signature gets extracted by the system. Sometimes there are data errors. Sometimes you may think there is a better representation of your fitness on that date than what the systems sees. Save/Lock is used in these situations (mostly to correct power meter errors).