1. Supply of wood:
The forests are made up of large amounts of trees, which, with regulated cutting, can continue to supply humans with wood needed for construction, fire-wood, ship-building (or at least before in history), and others.
2. Photosynthesis
The forests use Photosynthesis to take in carbon dioxide & monoxide (created by us humans), and with the process of photosynthesis, release Oxygen as it's waste (which we humans use).
~