Eating before Yom Kippur
It is a mitzvah to eat and to have a big meal (at least one) before Yom Kippur, according to the Shulchan Aruch. Big yes, but heavy, no; we do not want to arrive at shul for Kol Nidre feeling over-full, complacent or arrogant.
It is good to get home early and to break off other activities, even Torah learning, to do this. One reason for the mitzvah is to fill up for the fast. The Torah speaks of the ninth of Tishri as also being part of Yom Kippur (Leviticus 23:30); the eating that gets us through the fast is therefore part of its observance. Although I know people who've fasted on a sandwich, with no ill effects, it is not recommended. Another reason is that we want to make peace with one another on and people are less likely to start or continues fights on a full stomach!
Finally, Yom Kippur is a Yomtov, a day of celebration, and on Yomtov we eat. The snag, of course, is that Yom Kippur is also a fast when we may not eat. So the eating part of the Yom Kippur Yomtov is brought forward to the day before. The celebratory side of Yom Kippur is one that we tend to overlook. As well as being a serious time for personal reflection and transformation, it is also a joyful day of forgiveness, purity and celebrating our capacity, with God's help, to change.