Interesting data point. I'm assuming you wash your cars and don't "let the rain handle it?"
I don't park under trees enough to get any remarkable buildup of detritus in the channels with these weather strips in position. Though TBF my Toyota is comparatively younger than most here.
EDIT: I was curious, so I checked briefly with a search. FWIW, the most convincing answer I got was that it acts as a drip edge directing water away from the sides of the car and channeling it towards the back instead of the sides of the car. Other not so plausible stuff was that it would prevent leaks at the body seams (I don't find this likely given how easy it is to move our pieces, etc), minimizing wind noise, etc.