Drive Balancing tool is designed to "even out" the data on a drive pool for Windows Home Server. The main purpose was when a person adds a drive to the pool, it doesn't necessarily start getting used for a while. While Microsoft has made changes to the usage of the Landing Zone (as in it doesn't exist anymore), some people's systems might still be in a non "evened out" state as wish to correct this. On my own system, I routinely get within 2% of perfect balancing on all pooled drives.
Drive Balancer has 3 options. You choose (y/n) for #1 and then either 2 or 3:
1) Clear out D: (commonly referred to as the Landing Zone) prior to balancing data.
2) Balance data among pooled drives such that each drive will have an equal percentage filled.
3) Balance data among pooled drives such that each drive has an equal amount of data on it.
More info: Drive Balancing tool