Although it’s a long shot, a BOS would offer some protection to both parties:
1) For seller to prove he wasn’t in his possession if buyer robs a bank and leaves that handgun behind (that is if FBI could even trace the handgun’s chain of possession).
2) For buyer to have some evidence of the date and from whom he purchased the handgun.. IF that handgun was previously stolen.
Again, they are long-shot, near worst case scenarios and I personally don’t subscribe to using them.
As people have mentioned, unless it's notarized, anyone can create one anytime they like.