Rather depends on the van design, if the water spills over the side then it will deflect that away from directly tracking down the van side. For long term storage this could be useful, in particular if it "throws" the water clear of the window and its seals.
Vans featuring raised side edges, then the water tends not to track down the sides but over the ends. There, being on the side rail IMO it adds nothing of merit.
Our vans have been the latter, plus I almost always deploy an awning or Shadow sheet, and as said having a strip to add to the hassle is something I can do without.
The "Prof" has it. There is a whole lot of people making a living off "feeding" hobbyists, this hobby seems very good yield territory.