The office of the Vice President is a curious anomaly. The Constitution provides the VP with two functions: (1) to take over the Presidency in the event of the President's death, incapacity, resignation, or removal from office; (2) to preside over the Senate (something modern-day VPs very rarely do in practice). Other ill-specified functions have accrued to VPs in recent years, partly, I suppose, because Presidents (or VPs, or somebody else) started actually worrying about the idea of somebody suddenly becoming President without having any idea what was going on (see, for example, Harry Truman, who did manage to come up to speed pretty quickly).