Are those vent boxes or air intakes? My belief was that they are air intakes...?

If thats what they are, and we are talking about the strip that seals the "air intake" to the mudguard, it makes perfect sense to place them there. Without them, particularly in dusty or muddy environments, I can see dirt and debris being thrown off the track and potentially entering the air intake.

Completely assuming, if they weren't on earlier vehicles, they were likely something realized as being beneficial in certain conditions that were implemented as an improvement.