The theory is, you brown the meat to seal in the flavour, but as some TV cook pointed out if browning really did seal in the flavour then as the piece of meat heated and the moisture within the meat expanded as it heated up, sooner or later the piece of meat would explode, much as a baked potato can if there are no breaks in the skin when it is put in the oven.
I always thought the idea was also to caremalise the outside of the piece of meat to enhance flavour, which of course works well if you are cooking a steak or chop, but with a stew or casserole, once you add the liquid that will dissolve the caremalisation into the gravy.
Having said tthat I have always been far too lazy to brown meat. I put all the ingredients into a pan bring them to a boil and put them in my slow cooker. I doubt anybody can tell the difference.