It's not that the route isn't already known, it's so, just incase something does go wrong, you don't have to go routing around in your bumbag - at least that's why I carried my map in hand all the way. Someone I passed commented on me obviously being a newby as I had a map in hand (I didn't see him after that until he finished way behind), but it's all about preventing losses, rather than limiting them - although with weather like Sunday there's little danger of making mistakes!
I used my map to keep me focused when I started getting a bit bored on Brown Knoll.
I also presume running with a map-in-hand is more comfortable to those of us who Orienteer a fair amount

-
I know some people use the Pike of Blisco Omap on Langdale/Swindale North Omap on last year's Saunders.