The Ramban, at the end of Parshas Bo, writes that Hashem does not perform miracles in each generation merely to refute the notions of every nonbeliever who ever lives. Therefore, He commanded us to perform numerous mitzvos in remembrance of Yetzias Mitrayim - the era which He did perform countless overt miracles which disproved the possibility of any other supreme being, and demonstrated His continuous providence of all that occurs here on earth. According to the opinion of the Ramban, the purpose of many mitzvos is to help us achieve the level of emunah attained by Bnei Yisroel at the time of Yetzias Mitrayim.
On the pasuk, "And you shall relate to your children on that day saying, for this Hashem did for me when I left Mitzrayim" (Shemos 13, 8), Rashi explains, "for this" so that I should fulfill His commandments such as pesach, matza and maror. Rashi's explanation implies, in contrast to the Ramban, that the purpose of the mitzvos is not a remembrance of Yetzias Mitzrayim, rather the opposite, the very purpose of Yetzias Mitzrayim was in order that we should perform His mitzvos.
Rav Wolbe (Da'as Shlomo Geulah pg. 278) writes that in reality, the Ramban and Rashi do not disagree; rather, their explanations complement each other. Rashi in numerous places (see R"H 28a, Brachos 33b) writes that the mitzvos were given to us as a yoke, i.e. to show our absolute servitude to Hashem. The purpose of Yetzias Mitrayim was, as we say in Shema, so that Hashem "shall be for you a G-d." Bnei Yisroel were to accept Hashem's Kingship, which manifests itself by the performance of His commandments. The Ramban does not disagree, for he writes explicitly (Devarim 6, 13), "The [intent of a] mitzvah is to be like a slave who is acquired by a master who makes the work of his master primary and his own work secondary." The Ramban is merely giving a reason behind the mitzvos, as he writes, "And now I will reveal to you the ta'am (reason - lit. flavor) behind many mitzvos."
In other words, there are two aspects in every mitzvah. The idea of a mitzvah, as explained by Rashi, is to demonstrate our total subjugation to Hashem. Additionally, as the Ramban writes, each mitzvah has its own reason and purpose specifically designed to serve Hashem in that manner. The purpose of many of the mitzvos is to relive the spiritual high felt at the time of Yetzias Mitrayim.
Rabbeinu Yonah writes (Sha'ar Ha'avodah) that one is obligated to search for a reason behind each mitzva. Even if we fail to understand the reason behind a specific mitzvah, we must still fulfill that mitzvah as a servant fulfills his master's commandments regardless of whether he comprehends the motive behind their instruction. Nevertheless, if we do succeed in understanding the purpose of the mitzvah, it gives a wonderful "flavor" to the performance of the mitzvah. This being the case, it behooves us all to take a few minutes to review this most important Ramban (Shemos 13, 16 V'atoh omer lecha. . .) before sitting down to the Seder on Pesach.