The Kav Hayoshor (Chapter 97) brings down that the Magid revealed to the Beis Yosef (Magid Meisharim Vayakhel) an additional reason for Tanis Esther. There is a mitzvah to eat and drink on Purim, and all the drinking and merriment can cause one to sin if not done properly. Therefore, Hashem has us fast the day before Purim as a segula to protect us from sin, and the Satan and Lilis have no power to be mekatreg and bring us to sin. When one says in the selichos "shema tefila veha'aver tefila", he should have in mind that he shouldn't come to sin, chas v'Sholom through eating, drinking or Simchas Purim. May we all be zoche to real Simchas Purim.