As I understand it:

The object of washing is to remove the silver/hypo complex formed during fixing from the paper. When you start out with new fixer, there is no silver/hypo complex in the fixer; as it is used, the silver/hypo complex concentration builds up in the used fixer. This increases the level in your prints and makes it harder to wash the silver/hypo complex out of your prints. Using a hypo clearing bath makes it easier to remove the silver/hypo complex from the paper during washing, thus you can tolerate higher silver/hypo complex levels in your prints out of your fixer bath.