This question came up on Quora today, and I decided to answer it. Here’s what I submitted:
The superficial answer might be simply: It is important because it was the first place called a national park ever set aside by any country.
In answering this question one would have to ask two others: Important to whom? and Important in what way or ways?
Important to whom? Well, to anyone who cares about preserving remarkable landscapes from commercialism or from being despoiled. In 1872, when Congress passed the act setting aside the park and President Ulysses S. Grant signed it, the Yellowstone area was compared to Niagara Falls, because that phenomenon had not been preserved officially nor were businesses forbidden from setting up to sell whatever they wanted to the tourists who flocked there.
How was it important? Although no one in Congress had seen this remote western area, the men who had gone there were able to show photographs and paintings and tell them stories of what they had seen—phenomenal geysers and hot springs, lakes and waterfalls, mountains and valleys teeming with wildlife.
Not only Americans but people from all over the world are now able to visit and experience a place like no other, where only the necessary concessions are permitted and noone is trying to sell you anything outside of the few souvenir shops you may enter if you wish.