I've made many slow-cooked chicken casseroles without browning the meat first.
Anyway, Heston Blumenthal has looked at the science behind cooking meats and concluded that 'browning' or 'sealing' meats is always a completely pointless exercise.
However, it can be a good idea to "sweat" vegetables first - particularly onions, or you can be left with a distinct "raw onion" taste to your finished dish.