To go on, they're a result of two competing philosophies regarding natural areas and conservation from the early 20th century. The two schools of thought were essentially embodied in John Muir[1] (National Parks) and Gifford Pinchot[2] (National Forest Service). It's a really interesting narrative if you're at all interested in these distinctions and protected areas in the United States. It's the sort of thing that can be traced all the way through to attitudes today, and provides a nice context for why these systems exist.

[1] http://en.wikipedia.org/wiki/John_Muir#Activism_and_controve...

[2] http://en.wikipedia.org/wiki/Gifford_Pinchot#Forestry_policy...