Space Shuttle Endeavour was staged for a possible rescue mission in case Space Shuttle Atlantis gets into trouble while the crew is fixing the Hubble Telescope.
Because Atlantis will not be able to reach the space station if it gets into trouble, or is damaged, on its Hubble flight, the Endeavour orbiter will be made ready on the pad for a rescue mission in case it is needed.
Then, there is the LIFE biomodule, which we piggybacked in the Grunt mission simply to get it into interplanetary space for as long as possible, 34 months in the case of this mission.