It is because when it is precipitating outside your bag won't get ruined. I have ruined or nearly ruined so many beautiful leather or suede bags because I used them when it is raining outside. It doesn't really matter for people who go from a car to the office but for those of us who walk to class or work it matters a big deal. Also the price tag doesn't induce cardiac arrest as much as most bags. So here is a post on popular nylon bags floating around. I own the Burberry one! LOVE IT!
Burberry Nylon Shopper $250 plain $295 plaid print
Marc by Marc Jacobs Tate Toe $178
Longchamp Le Pliage $145