This song is just so amazing.
The fight with Yami is pretty darn intense. While not particularly difficult, the fight still has a very bleak, almost hopeless feeling. You start with basically no powers, and have to struggle against him for the majority of the fight, as you slowly gain your abilities back...but at the same time, Yami only continues to grow more powerful as well.
Then, he strikes you down. The moment you let your guard down, you are left wounded, powerless, and at the mercy of Yami...but then...the entire world calls out to you. Everyone you've met, everyone you've helped...they all come to your aid, they share their power with you...and with their prayers, wishes, and hopes, you gain all your power back and then some, as THIS music starts.
The entire world is on your side, lending you their strength...and with that power, you can now burn away the darkness, and CRUSH the dark lord like the bug he is! That is why this song is so great...while it is an amazing song by itself, the way it is used in the game is just incredible. After the power struggle the boss fight was until this point, this music screams "You have the upper hand(or paw)! Yami is nothing before you now!" It's so empowering, and just makes you feel like you CANNOT lose!