Why it impacts your cost to reach people
Put simply, the higher an ad's relevance score is, the less it will cost to be delivered on Facebook.
This is because Facebook's ad delivery system is designed to show the right content to the right people, and a high relevance score is seen by the system as a positive signal.
Of course, bid matters too. But, overall, having strong relevance scores will help you see more efficient delivery through Facebook's ad system.