It's about the best balance of water holdout and breathability.

Moisture control in the wall cavity is essential to reducing the risk of moisture damage. It’s important to protect the wall cavity during construction and after the home is complete. TYPAR^® HouseWrap acts as the primary line of defense against the elements during construction, and a secondary water barrier after the cladding is up. But moisture can enter the wall cavity from inside the home as well. TYPAR’s optimal perm rating (11.7 US perms) allows the wall cavity to breathe. So moisture vapor can escape the wall cavity. In addition, the TYPAR material itself does not support the growth of mold unlike traditional building papers.