Fishers Peak is the highest point in the United States east of the approximate north-south line formed by Interstate 25, the highway that follows the eastern base of the Rocky Mountains through Wyoming, Colorado, and New Mexico. It lies just east of well-known Raton Pass, which has been a historic route since long before they built I-25 through it. Therefore, this obscure mountain has a good claim to being the highest peak east of the Rockies, an honor more often claimed by Harney Peak or Mount Mitchell, both well-known state high points.
And, if I-25 in Colorado and northern New Mexico is the western boundary of the Great Plains, then Fishers Peak is the highest point in the entire "North America Plains" range area, a huge chunk of the continent.
Sierra Blanca, a 12,000 foot peak in southern New Mexico, is east of I-25, but that far south there are no Rocky Mountains to be east of. And at that point I-25 has taken a big jog to the west and no longer follows the same general north-south line it does north of Las Vegas, NM.