Once reason people may hide their license plates from view in a video or photo online or is they may not want people to know what State they're living in.
Although just knowing the state is still hard to find the person.
Other than that there's no other good reason to actually hide license plates and license plate numbers from view in a photo or video since nobody but the police, DMV, etc have the means of finding the owner of that License plate.
There's no online service for the United States that allows regular non official employees of the DMV or police to search license plate numbers and get an address.
I don't hide my license plate in photos or videos and I don't ever plan too because there's really no reason for me too.