Why is the haggadah important to the seder meal?

This meal, called the Passover (Pesach) Seder, is of great importance in Judaism. It is a 3325-year old continuous tradition that began on the night of the Exodus from Egypt (see Exodus chapter 12), and is fully detailed in our ancient Oral Traditions (Talmud, chapter Arvei Pesachim).
The Seder meal is one of those occasions, like Yom Kippur and Hanukkah, that Jews all over the world, Orthodox and non-Orthodox alike, observe in common. During the Seder, we keep the essential mitzva and customs of handing Jewish traditions down to the next generation, with the traditional Seder foods and the ceremony of reading the Passover Haggadah which retells the events of the Exodus.
During all the days of Passover, matzah (unleavened bread) is eaten; while leavened foods such as bread, cake, cookies, cereal and pasta are forbidden.
During the Seder meal, there are other traditional foods as well: bitter herbs, wine, parsley, and haroset (a mixture of Apples, cinnamon, wine and nuts).