It's effectively a tool to show you how good the chances you are creating actually are.
You could have 23 shots in a game and lose to an opponent who had 2. Looking at it purely as number the team that had 23 shots should have won. Look further in to the stats and all 23 could have been speculative efforts that are never going to score whereas the 2 for the opposition were in better positions etc.
That's a very basic explanation of it.