It is just another way for you to add consistancy to each and every shot. Almost like an anchor point. It does matter if you cant your bow, especially at longer yardages. Say you cant your bow to theright for a right handed shooter, (top limb tilted towards the right), it will move your pin up and to the right from the level position. If you are sighted in at level, this cant will cause you to shoot low and left.

Years ago, when I shot fingers release and I first put a sight on my bow, I noticed that my 4 sight pins were angled to the right when I held the bow level. They looked like this.
0 20
030
0 40
0 50

The top pin was furthest left and the bottom pin furthest right. I hope this diagram stays intact when I post this. But anyway, the level is to help the archer keep his bow level while executing shots and help to consistantly keep the pins in the same location.

I hope this helps.