Final answer:
The Gateway Arch is pivotal for its status as an architectural triumph, its symbolic representation of westward expansion, and its innovative engineering reflecting a catenary curve.
Step-by-step explanation:
There are multiple reasons why the Gateway Arch is an important landmark. Firstly, as a monument of Eero Saarinen's architectural accomplishment, it represents a significant achievement in modern design and construction, being the tallest stainless steel monument in the world. Secondly, it serves as a symbol of the city of St. Louis and a nationwide symbol of westward expansion in the United States, reflecting a pivotal moment in U.S. history. Finally, with its unique structural form based on the concept of the catenary curve, it's also an example of architectural innovation and engineering expertise, offering visitors unique views of the city skyline and the Mississippi River.