Answer : Occurs only when there is an error in it
You must have seen it in somes/w like uTorrent (A movie downloading s/w) . it has the facility of check pointing means in middle of downloading if you loos the internet connection (Kind of error for software) then it stops the movie exactly where it is(makes a check point there) .It does not start downloding it from starting once you get your connection back .That why this option makes sense ..
