I was reading Wiki about filters and saw this:
Some lenses like Canon's L series lenses require the use of a filter to complete the weather sealing
Obviously Wikipedia has to be taken with a grain of salt so I decided to dig and see why they would post that.
From an old PotN post I found this:
Here's what the 16-35/2.8 II manual says:
"Although the lens is dust- and water-resistant, a Canon filter must be attached to the front of the lens for complete protection. Unless a protective filter is attached, the dust- and water-resistant features are not assured"