Yom KippurYom Kippur, the Day of Atonement, is the tenth day of the month of Tishrei. Yom Kippur is the holiest day of the year. On Yom Kippur, we spend the whole day praying to Hashem for forgiveness. Keeping with the holiness and importance of the day, the restrictions on Yom Kippur are greater than on any other day during the year. On Yom Kippur, besides observing the regular restrictions that Shabbos carries with it, there are six added restrictions. It is forbidden to eat, drink, wash one's self, anoint one's self, wear leather shoes, and engage in marital relations. Why do we have these restrictions?
The Sefer HaChinuch explains that our having Yom Kippur is a kindness of Hashem. Hashem established that all the creations of the world have one day during the year on which, if they repent, their sins will be forgiven. Because of the focus of the day and how important it is that we stay focused during the day, we have these added prohibitions. Eating, drinking, and other sensory pleasures may entice the "earthly" (or non-spiritual) element in a person to be further drawn towards sensory pleasures and indulgences, and sin because of that. Furthermore, if a person allows himself to be caught up in sensory pleasures, he will have a barrier placed in front of his intellect, that will prohibit the intellect from striving for the truth. The "truth," of course, is serving Hashem and adhering to His Torah.
One must also remember that on Yom Kippur, we are like slaves pleading for mercy in front of our master. It is not sensible for a slave who is standing in judgment before his master to have a less than clear mind, so that he can present his case, defend himself, and plead for mercy properly and effectively. It is therefore not fitting for this slave to have a less than whole heart on the matter because he is concerned with food and other "pleasures" which only serve to confuse and distract the slave from the important issue at hand. We, the slaves, have to assure that we stand before Hashem with our minds totally focused on repentance and coming close to Hashem on this day. Having our minds distracted by other "earthly" matters, especially sensory pleasures, will only serve to harm us and prevent us from doing that which we need to do now. Therefore, it is good for us to strengthen our intellect and souls, and to suppress our earthly desires on this hallowed day, in order that we should be in the proper state of mind. If we keep our focus on what the significance of the day is and what our goal is for the day, it is very likely that our atonement will be accepted. We must make sure that our earthly desires do not cause our downfall. As our conduct on this day concerns a matter of life or death (specifically, ours), it is of utmost importance that we do not let ourselves get distracted and perform less than wholeheartedly.
For the above reasons, we have the additional prohibitions. The Sefer HaChinuch adds that these are also reasons why we keep the probitions of Shabbos on this day. We are forbidden from performing the acts of "work" prohobited on Shabbos so that we will not be busied with anything else on Yom Kippur, and hence be distracted from repenting properly. We will be able to place all our thought and our heart into requesting forgiveness from Hashem on a day that, since the creation of the world, has been set aside as a day of atonement for our sins.