If on concrete and int he event of a negligent discharge the bullet could splatter shrapnel and cause injury to those in the vicinity. (Most ranges have unloaded and empty actions to prevent this)
If there is something sharp and hard enough on the ground you could screw up your muzzle crown.
Some debris could cause a barrel obstruction leading to ruined firearms and the risk of chunks of metal lodging in bystanders.
Just some thoughts on the reasoning behind the request. Take it for what it is (speculation). I'm partial to the second one myself and would be quite irate if someone took my Kidd 10/22 and decided the muzzle should rest on the floor...
