You may have heard that sealing meat before cooking helps to lock in flavour and juices. This is, in most cases, untrue. It does however help to add extra flavour. Browning the meat over high heat creates a chemical reaction that actually intensifies flavour and aroma. (I won't go into the science of this, but think about a piece of sliced bread vs toast).
Your joint will be perfectly edible if you skip the browning process, but you'll be missing out on a load of flavour. Plus, it helps to add a bit of a crusty surface, adding to the texture of the roast.