It's important for all the above reasons...litmus test, confidence builder, and this is THE swing game to gauge whether or not we will have a winning record in the division this year. The Niners are tied with us for record, and this would knock them behind us, while also giving us the edge in a tie breaker with the head to head.
Also, we really need to serve notice to the division that our defense is here. The Niners served notice last year, now it's our turn.
The other nice thing is that the Niners have 6 out of their last 10 games in division. With the physical defenses our division has, they will definately be beat up at the end of the year. If we outpace them now, they will be playing catch up, and might be too emotionally drained to really give it to us in our house at the end of the season.