The theory is that browning seals the flavour in. If it did the meat would explode as its water content in the meat expanded with heat and couldn't escape because it was sealed in. More to the point it caremelises flavour on the outside, however, as you then slow simmer the meat etc for many hours, it all just dissolves into the gravy. So why bother?
I stopped wasting my time and energy browning meat or vegetables for casseroles and stews, decades ago. Put it directly into the slow cooker and let it get on with it.