When cooking stews and casseroles, recipes will say to brown the meat in small batches before adding the other ingredients. I have often been asked why do we brown meat, and why brown it in small batches?
Searing the meat in a very hot frying pan caramelises the surface of the meat. This in turn enhances the flavour of the meat, giving it a caramel/nutty flavour. The best way to brown meat is in a really hot pan and not putting too many pieces in at the same time. If the pan is not really hot, when you add the meat the temperature of the pan will decrease and you will not sear the meat. If there is too much meat in the pan, again the heat will decrease and you will end up braising or stewing the meat and not get a lovely brown colour.
If you miss out the browning stage you may end up with a little less flavoursome casserole. However, skipping it will not cause a disaster. I have sometimes skipped it myself if short of time, or if I just cannot be bothered!