Why is there Passover?

Answer 1

Because the Torah commands it (Leviticus ch.23).

Pesach (Passover) is important to us since in it we relive the Exodus from Egypt and our birth as a nation, both of which were preparations for receiving the Torah from God.
The highlight of Passover is the Seder meal. This meal is of great importance in Judaism. It is a 3325-year old continuoustradition 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 the Seder meal, other traditional foods are eaten in addition to the matzah: bitter herbs, parsley, wine and haroset (see below). Salt water, a roasted egg, and a bit of roasted meat are also on the table.
During all the days of Passover, matzah (unleavened bread) is eaten; while leavened foods such as bread, cake, cookies, cereal and pasta are forbidden. Certain prayers are added in the synagogue services, and the Torah is read each day.
Here is the symbolism of the items on the Seder plate:
The bitter herbs (maror) symbolize the harsh slavery which the Israelites suffered in Egypt. Horseradish and/or romaine lettuce are traditionally used for maror.
Charoset - A sweet mixture representing the mortar used by the Jewish slaves to build the storehouses of Egypt. In Ashkenazi Jewish homes, charoset is traditionally made from chopped nuts, grated apples, cinnamon, and sweet red wine.
Karpas - A vegetable other than bitter herbs, traditionally parsley, which is dipped into salt water at the beginning of the Seder. The dipping of a simple herb into salt water (which represents tears) recalls the pain felt by the Israelite slaves in Egypt.
Beitzah - A hard-boiled egg, symbolizing the korban chagigah (festival sacrifice) that was offered in the Temple in Jerusalem and roasted and eaten as part of the meal on Seder night. Although both the Pesach sacrifice and the chagigah were meat-offerings, the chagigah is commemorated by an egg, a symbol of mourning.
Z'roa - A roasted lamb or goat shank-bone, chicken wing, or chicken neck; symbolizing the korban Pesach (Pesach sacrifice), which was a lamb that was offered in the Temple in Jerusalem, then roasted and eaten as part of the meal on Seder night.

See also:

How is the Seder celebrated?

What is the importance of the Israelite Exodus?

Answer 2

Passover, or Pesach in Hebrew, is one of the most sacred Jewish Holidays. It commemorates the redemption of the Israelites from their slavery in Egypt, and specifically the occasion when the Angel of Death passed over the houses of the Israelites.

Notes: Afikoman means 'dessert' and the use of the word is a modern addition to the Seder that was added during the medieval period. Additionally, the matzah we eat today, square and perforated so that it looks like it is striped came about in the late 1800's when the Manischewitz company invented the first machine to mass produce matzah. The mato is unleavened to remember how, when fleeing Egypt, the Israelites did not have time to let their bread rise and instead baked it into flat disks.