The block girdle helps with NVH at high rpm, contributes to block strength and specifically helps with main bearing and crankshaft life at high rpm.
It ties all of the main caps together producing a stronger bottom end. None of the main journal supports can move independently of each other and shake around. (It's even better to tie the girdle into the outer block skirt but that's not something our engine has. See the Honda F20C.)
It does also slightly hinder oil return to the pan and promotes windage. I think Miko has said in the past quite a bit of power can be regained by removing the girdle because of the reduced windage. I've always preferred having the girdle versus freeing up that power.
They are extremely common on race engines for obvious reasons. (Then again race engines don't have to worry about windage because of dry-sump oil pumps.)