The Hasting's Cuttoff was an alternative road for westward travel for the immigrants who were making their way towards the California. The reason James reed and George Donner took this alternative route was because it was a shorter way.
Their decision was definitively unwise as the the trail, before their journey, was not tested and thus the undertaking was severely dangerous. Unfortunately their party got trapped by heave snowfall as many of the survivors had to resort to eating to dead in order to survive.