By removing less desirable and low-grade trees, you make room for healthy growth. You also create a financial incentive for good forest management, instead of indiscriminate cutting that takes out all the good trees and leaves the culls behind. A typical logging operation leaves as much as 40 tons of wood behind on every acre. One ton of wood is equal to about 40 gallons of oil, so that's the equivalent of 1600 gallons of oil per acre that would go to waste if it weren't for the wood plants. Use of wood-fueled power helps improve our forests, creating environmental benefits over and above the obvious economics.
Good forestry is important for water quality, because it prevents erosion and sedimentation.
Trees also purify the air we breathe by absorbing carbon dioxide and releasing oxygen. And consider the aesthetics of forests. There's just no way to calculate their value. The added long-range benefits that it provides to us from improved forests make it a unique opportunity.