Searing over high heat caramelizes the surface of the meat, which enhances the savory 'meat' flavor and fills the finished dish with complex layers of nutty caramel and coffee-like bitterness. In technical terms, this is called a Maillard reaction and it's a flavor profile we omnivores happen to find quite delicious. Without searing, meat dishes can taste flat and boring.
Admittedly, searing isn't strictly necessary for the cooking process. Technically speaking. The meat will cook just fine without searing. (And any surface bacteria will die during cooking anyway.)
But I really believe that the depth and complexity of flavor we gain in this searing step is well-worth the extra effort. What about you?
(Image: Individual Pot Roasts/Faith Durand)