The Torah informs us in this week's Parsha about the Mitzvah of וידוי, confession of one's sins (במדבר ה;ו,ז'). We learn that if a Jew has transgressed a מצות לא תעשה, a negative commandment, or has failed to fulfill a מצות עשה, a positive commandment, and he regrets his action or inaction, he has the opportunity to do Teshuvah. At the beginning of הלכות תשובה, the Rambam teaches us that true Teshuvah has three parts. First, the person must admit that he has done something wrong. Then, he must sincerely regret that which he did. Finally, and in a way most important,he must make a firm commitment to himself not to do this thing again. The וידוי, the oral confession introduced in this week's Parsha, incorporates all three of these ideas; the וידוי, then, is a very central part of the Teshuvah process. When reciting the וידוי, the sinner pleads with Hashem, saying that he has sinned, that he is sorry for his action, and that he will never do it again. This puts him on the road to Teshuvah.
In the time of the Beis HaMikdash, one who sinned had to say this וידוי before bringing the Korban Chatas which brought him atonement. But even without the Beis HaMikdash, we too still have to announce and confess our sins because we must show that we understand that Hashem gives reward and punishment and because by saying the words of the וידוי, we will better be able to remember not to do the sin again. Chazal thus tells us that if the Jew does proper and sincere Teshuvah, Hashem will act with kindness towards him. Moreover, we are taught that whoever does Teshuvah and says וידוי before he dies will have a portion in Olam Habo. We therefore see how important a part of the Teshuvah process וידוי is, and can now perhaps understand why וידוי is such an integral part of our davening on Yom Kippur.