Pretty much what it says on the tin. Look at past men and women who plausibly could have been PotUS, either through election, or through the President dying at the wrong time (FDR was pretty unhealthy for all his terms), and would have done a horrible job at it (to continue the FDR example, I have little faith in Henry Wallace, who was VP for a while during WWII).
Its here because we had Presidents well before 1900, even if the example was after.