The gemara learns out from the pesukim that there is a mitzva to eat on Erev Yom Kippur. What is the nature of the mitzva?
Rashi and The Rosh in Yoma both learn that it is to prepare for the fast. Other Rishonim learn that it is an independent mitzva to eat, independent of the fast. There are a number of nafka mina's brought down in the acharonim:
1. Are women חייבות in this mitzva? If it is to prepare for the fast then yes, just like they are obligated to fast they would be obligated to prepare. However, if it is an independent mitzva then it is a מצות עשה שהזמן גרמא and therefore they would not be obligated.
2. Is there a mitzva to eat the night before? If it is preparing for the fast no, eating the night before is not preparation. If it is an independent mitzva then yes.
The Gra brings an interesting proof that there is no mitzva at night from a gemara in Kesuvos 5a. The gemara there discusses why it would be prohibited to get married on מוצאי שבת. One reason offered is that since you are going to have a seuda Saturday night we are worried that you will inadvertantly be mechallel shabbos and prepare for the seuda on Shabbos. The gemara asks if so how do we allow Yom kippur to fall out on Monday? We should have the same concern about the seuda (that you will prepare it on Shabbos). The gemara answers that on Erev Yom Kippur יש לו רוחה, he has time. Rashi explains because the seuda is only the next day during the day. We see from here that there is no chiyuv to eat at night.