As the scope of responsibilities of the President of the United States increased during the 20th century, the Vice-President came to be relied upon to handle greater responsibilities also.
A 21st century Vice-President, Joe Biden, said it well: “The way the world has changed, the breadth and the scope of the responsibility an American president has virtually requires a vice president to handle serious assignments, just because the president’s plate is so very full." Biden was quoted in the book, The American Vice Presidency: From Irrelevance to Power, published by Smithsonian Books in 2014.
Another point might be made about 20th century Vice-Presidents -- especially in the latter half of the century. A number of Vice-Presidents became President because of things that happened to the President. . President Franklin Roosevelt died while in office, and was succeeded by his Vice-President, Harry Truman (in 1945). Vice-President Lyndon Johnson became President when John F. Kennedy was assassinated in 1963. Gerald Ford rose from Vice-President to President when Richard Nixon resigned to avoid impeachment.
In the current situation of American politics, with partisan politics strongly dividing Congress, Vice-President Mike Pence has functioned a number of times as the tie-breaking vote in the Senate on important matters -- another way the Vice-Presidency has taken on greater responsibility lately.