Technically, you do not have to brown roast before you add it in the slow cooker but I HIGHLY recommend it.  If you don’t sear your roast beforehand, you will miss out on the complex caramelized flavor that comes from the deeply golden crust.  You can’t fake this rich flavor any other way.  Searing the chuck roasts first also helps protect it from drying out during the long cooking process by sealing in the juices.  Once you sear your meat, you will never cook it any other way!