Pie is a pretty good card. When used, it will make a monster immune to damage until your turn ends. It also gives the monster Haste. This is extremely useful to use on Papyrus, as he can kill the entire enemy board without taking any damage (despite not benefit from Haste).
It can also be useful to kill cards like Mettaton NEO or Tsunderplane that are left on the board, preventing their Dust Effect hurting or killing your monster. (Keep in mind that you will still take direct damage.)