Diapers advertise that they can last up to 12 hours because they are saying that the disposable diapers your baby or toddler or kid wears should last up to 12 hours during the night.
The 12 hours of protection on diapers does not mean that it will last 12 hours during the day and you shouldn't leave the same diaper on your baby, toddler or child for 12 hours during the day as they will often pee more through the day or even poop the diaper, which will require changing your baby or toddlers or child's diaper every 2 to 3 or 4 hours.
Overnight protection is the main issue.
As babies and toddlers grow and get older there's comes a point when some babies and toddlers will start sleeping all night long and might be in the same diaper for 8 to 10 hours when they do sleep all night long.
By saying "12 hour protection", parents feel like it will be a good choice for overnight.