One possible answer is that someone could claim ownership of that firearm if they have the complete serial number. Example being, not paid in full, stolen or they say it was on loan and want it back. Some people will go to great lengths to just cause misery and make a fast buck off decent folks.
I just do it out of habit. I've been editing out serials ever since I sold guitars. Once, before I edited out numbers, somebody used my photos of one of my guitars to create a fraudulent eBay auction and scam some poor guy out of a cool grand. Since then, I've been editing out numbers. Hard to claim a guitar is yours if you don't know anything about it.