Insurance
It's simple. How can you prove what you have to your insurance in case of fire or loss unless you have a list? Maybe you can remember most of them, except that one you picked up cheap at a yard sale or that one still in the box you won in a raffle. With a good list of the firearms you own, it will help ensure that you are compensated for your loss. That is, as long as your insurance company isn't completely crooked.
Theft
You might have the boxes from some of your guns that you can get the serial number off of if they get stolen. But do you have them all? Probably not. Being able to report serial numbers to law enforcement will help them recover your stolen guns from the criminals that took them. With the serial number reported, you might even get it back someday.
Estate
Remember you late great-grandfather's house with a gun in every corner. Maybe one in the barn, one hidden in the bedroom, by the back door, or under the seat of the truck. I have heard dozens of stories of people finding guns in vehicles or on properties they've bought. Guns that could have gone to an heir but instead ended up in the hands of a random person. Keeping a list will let your heirs know exactly how many guns you have stashed around like your great-grandfather. Adding the values in as well, will help guide them in the sale of the firearms if they're not a gun nut like you.
So you should record the serial numbers for yourself, for your insurance company, and for your heirs. But just like the back of the book says, "If the feds ask, these were all lost in a boating accident."