It's actually easier to see why wingers do so vs wigan more than most. Assuming your talking about the first Wigan try, wigan had the ball player, the centre flat, the fullback behind him, and the winger wide - percival is covering the flat pass and that leaves the ball player 2 options - either pass it flat to his winger or centre and hope they beat their opposite 1 on 1 or swing it behind to the fullback and hope to expose the winger in a 2 on 1.
This gives Dawson a choice, he either stays on the winger guarding the flat pass, or he tries and catch the fullback as he catches it to stop the 2 on 1 - in the try you refer to he makes the wrong choice and goes to Bowen - leaving the wing open - however the magic weekend has 2 very good examples of wingers calling it right and cutting out the overlap