One of the unique aspects of the Seder is that we interrupt the saying of the Hallel with a meal. Why is that? The Netziv explains as follows: The purpose of going out of Egypt was to receive the Torah. With the Torah we gain the ability to serve Hashem not only through "spiritual" means, such as Torah study and prayer, but through "physical" mitzvos as well, such as marriage, enjoying Shabbos, eating matza, marror, and the Pesach offering. We eat in the middle of Hallel in order to praise Hashem for sanctifying and elevating our physical existence. Even "mundane" things like eating are elevated when we do them in the service of Hashem.
[from Rabbi Mordechai Perlman, Special thanks to Ohr Somayach Institutions www.ohr.edu for the article]
