I have never considered them to offer any practical advantage advantage. I've never had a channel get blocked with debris, and they won't stop the channel corroding, and if left in for any length of time could become difficult to remove if corrosion starts.
The only thing they may do is to direct water drips further from the side wall, but if there is even just a slight breeze it won't make a big difference.
I class them as another product to beguile caravanners out of more money.