The importance of seniority in the United States Congress can not be underestimated. Seniority is determined by which of each state's two Senators was elected first. The first elected is the Senior Senator, the other is known as the Junior Senator or bitch.
Senior Senators are really old men (and occasionally women) who have spent their entire working lives in politics and have grown so accustomed to sucking on lobbyists' teats that it is better for them to remain in office until they die.
They are also given primo seating in the Senate chamber, which they rarely visit, and have position over the Junior Senators from their states when it comes to decent seats at sporting events and the like.
Senior Senators are also given plum committee chairs or Chairmanships, which allow them to further develop their connections to lobbyists like Jack Abramoff.