If one looks at Pinchots disagreements with Muir it's pretty easy to see that he was a conservationist and not a preservationist. After all Pinchot was a professional forester.
Reading this it pretty clear. And the water sheds part pertains to its ability to grow timber.
www.fs.fed.us
I would suggest researching the Forest Reserve Act of 1891 and the decade or two leading up to it. It was not signed into law for the logging industry.
After that act was passed, some years later the Forest Reserves were transferred from the department of interior to the USDA under Pinchot and renamed the USFS.
Then there was a massive inventory of sorts that took place, led by Pinchot. Mapping, trail building, etc.... Resources were assessed rather than just blindly deforesting areas with no understanding of the ecosystems, Forest size, Forest types, water sheds, etc.
You stated the USFS was originally created for a sustainable source of lumber. You're wrong(it's ok it happens to all of us). As I stated above, it was much more broad than that. The original discussions that lead to the division of forestry were discussions and concerns based in a fear of exploitation and misuse. They wanted to prevent that. Pinchot wanted to prevent that.
"The reserve system, established in 1891, was "intended to stop unregulated timber harvest on sensitive public lands", Pinchot believed that the reserves then under the control of the Department of Interior- should be transferred to the Department of Agriculture because Interior was fraught with mismanagement and political infighting. Pinchot maintained that the small Division of Forestry in the Department of Agriculture, which already dispensed advice to private and public timber interests,was best suited to administer the reserves because it would not "bow to political pressure or Big Money." He told Congress that "in the administration of the forest reserves it must be clearly borne in mind that all land is
to be devoted to its most productive use for the permanent good of the whole people. . . .""
The USFS is the regulatory arm to manage that resource in that fashion...timber included, water sheds, wildlife, grazing, etc...to prevent exploitation of the land.
www.wilderness.org
www.fs.fed.us
www.fs.usda.gov › 2015/06 PDF The USDA Forest Service— The First Century
Link to "The forest must come first"