The Rishonim view the obligation to eat a seudah on Erev Yom Kippur as a means of preparing properly for the fast or as a pre-emptive celebration of the simchas Yom Tov of Yom Kippur, when we cannot eat, by having a meal beforehand. Both of these explanations see the celebration of Erev Yom Kippur as an extension of Yom Kippur itself, but not a day with intrinsic significance.
The Sefas Emes (5662), however, offers another perspective. During the failed attempt to receive the first luchos, Bnei Yisrael eagerly anticipated Moshe’s return for 39 days. With each day’s increased anticipation came an increase in the effort of the yetzer hara to somehow undermine events. It was at the last minute, on the last day, the yetzer hara unleashed its final most powerful push, its "full court press", and caused the confusion that led to the tragedy of the eigel. This is often how the yetzer hara operates – it allows events to unfold, it lulls us into complacency, and then just as we reach the finish line, all sorts of obstacles and challenges appear and cause us to question and even to abandon all the effort and good work put in until that point.
We are now at the finish line before Yom Kippur, the commemoration of the giving of the second luchos. The yetzer hara tried again at that time and tries again every year to undermine all that we have accomplished from Rosh Chodesh Elul until this moment, because he knows that all it takes is a slip on this final day, another eigel on that final day, and all the effort put in until that point will prove for naught.
Erev Yom Kippur is a day of celebration in its own right because on this day the second time around there was no eigel – we beat the yetzer then; we can do it again now.
Gmar chasima tovah to all.