As I wrote here, one of the issues for the U.S. Supreme Court, early in its existence, was not having enough work to do. In a small country, with a decentralized political structure, the caseload was minimal.
By the latter part of the 19th century, the country had grown, and the scope of federal law had grown. A series of actions were taken to help the Supreme Court deal with its increased caseload.
In 1891, Congress established the federal Courts of Appeals, organized into nine "circuits". Today, there are 13 judicial circuits.
The circuit courts hear appeals from lower courts. Consequently, the Supreme Court's role has largely been reduced to considering issues on which different circuits have issued contradictory opinions.
I addressed two changes in the first half of the 20th century, in this post. During William Howard Taft's tenure as chief justice, he advocated two changes to which Congress agreed. One was to allow the Supreme Court to decide which appeals it would consider, and which it wouldn't. Another was to build a separate building in Washington for the Supreme Court; before the completion of that building in 1935, the Court met in the Capitol.
The Supreme Court building is designed in such a classical style, it's easy to believe that it goes back to the original plan for Washington, DC, along with the headquarters of the other branches, i.e., the Capitol and the White House. But it was built more than a century after those buildings.