There are a variety of benefits to be able to corelate information accross different systems -- RADAR returns aren't perfect, I think you can get added situational awareness from comparing the 'known' from the 'apparent' (i.e., return) position.

Seeing your "route" with targets moving accross it helps sometimes see things you wouldn't otherwise easily understand.

Another factor to consider. When you switch from North Up to Course Up in situations, having the rout overslaid provides instant visual confirmation of the status.