To protect yourself from the rain
Bad weather can quickly turn your journey into an absolute ordeal. There's nothing worse than driving around and arriving at work completely drenched. The poncho offers the advantage of protecting his backpack, the top of the legs and of storing/folding in his bag easily.
If you don't like the style, you can opt for a rain jacket or tell yourself that you will look (a bit) like Clint Eastwood.