Certainly William Henry Jackson's photos of Yellowstone changed the minds of Congress and were a major influence in creating the first National Park. They provided the irrefutable proof that the tall-tales of the Yellowstone area were true.
For me personally, I think Weston's Pepper #30 did more to alter my perception of what constitutes "an important subject" than any other photo. It gave me the idea that no subject is too small, mundane, or unimportant - it was only my failure to see the intrinsic value within the subject. Something I've been working on ever since - trying to see the importance of things that surround me.