Why do we use a roasted (maybe?) egg on the Seder Plate to represent the Festival offering?
The Shulchan Aruch Harav says that one eats eggs to remind him to mourn the Beis Hamikdash, since had the Beis Hamikdash stood, we would have been eating the korban Pesach.
The Taz (OC 473:4) suggests the reason is so that we can eat it, because it is forbidden to eat roasted meat on the Seder night.
The Mishna Berura (:23) quotes two other reasons. First, that we use an egg, which is commonly served at a meal to mourners, to represent our mourning the loss of the Beit HaMikdash and the Korbanot. Second, the word for egg in Aramaic is ביעא which is symbolic of the phrase בעי רחמנא למפרק יתנא (a plea for redemption).