I was once told that the idea behind covering your outboard is that they aren’t designed for the wet road driving environment of fine sand and salt that we have here in canada. One drive down the highway in the rain and you all know the horrible thin layer of road grime that coats absolutely everything.
a car’s air intake typically has a pile of bends/collection points plus a large air filter for this kinda junk, meanwhile my personal outboard has a small piece of foam before the intake and that’s about it. Hence, I cover the motor to try and keep some of that road grime out of the intake and engine bay in general.