FPI does it, Bill Conelly added it to S&P+, etc.

When you have 12 games that are mostly not related and they only play each other once trying to map, rank and predict 130 teams is REALLY hard.

Adding in previous years is helpful because those players are returning. It gets weighted lower adn lower as the year goes on, but you dont start with a blank slate every year - problem areas are rarely fixed by one offseason.