It stops people sitting on a high rating without playing any games. For example someone who has the crown in a game with 750 points will lose 5 points a day to prevent them from staying on that score and retaining the crown without playing. 

Below is the system FOD uses to determine the amount of points you will automatically lose each day:

Below 353 1 
354-500 2 
501-612 3 
613-707 4 
708-790 5 
791-866 6 
867-935 7 
936-1000 8