One of the Mitzvot which we perform on the night of Pesach is the Mitzvah of Sippur Yetziat Mitzrayim. The outline and form through which this Mitzvah is performed is given to us in the Gemara (Pesachim 611a) -- we start our discussion with the bad, and then proceed to discuss the good, מתחיל בגנות ומסיים בשבח. There is a Machloket there as to what exactly the bad is that we start from. Rav explains that we start off by telling how our forefathers worshipped Avodah Zarah while Shmuel says that the bad is our enslavement in Egypt. In our Haggadot we incorporate both of these opinions by saying עבדים היינו, we were slaves, and מתחלה עע"ז היו אבותינו, at first our fathers worshipped idols. However, what is not discussed is what constitutes the good which is supposed to follow.
It would make sense that according to Rav, who says the bad is our forefather's worship of Avodah Zarah, the shevach would be the next line in the Haggadah, ועכשיו קרבנו המקום לעבודתו, and now Hashem has brought us to worship Him. However, what is the good according to Shmuel? The Brisker Rav suggests that this phrase about our accepting Hashem is also the shevach even according to Shmuel. The whole purpose of Yetziat Mitzrayim was for us to gain the status of Avdei Hashem, so that we could not be Avdei Bnai Adam anymore. This is why the Torah says והוצאתי אותם מארץ מצרים להיות לכם לאלוקים (Bemidbar 51:14). So according to Shmuel, who says the bad is our enslavement in Egypt, the good can still be our becoming servants of Hashem since that is the only reason why we were freed. (See, however, the Rambam (Hilchot Chametz U'Matzah 7:4 who says that the shevach according to Shmuel is telling the about the miracles and wonders Hashem did for us when we were taken out.)
This also explains why Rav said the bad was our forefather's worship of idols. It seems absurd to start the Mitzvah of Sippur Yetziat Mitzrayim with idol worship since at first glance it has nothing to do with Mitrayim or our freedom from it. However, with the understanding that the whole point of Yetziat Mitzrayim was to gain the exalted status of being Avdei Hashem, it makes sense. Before Yetziat Mitzrayim we didn't have the status of being Avdei Hashem, and as such our forefathers were idol worshippers. However, after we were freed from Mitzrayim, we were able to become Avdei Hashem.
It is also for this reason that we make mention of Yetziat Mitzrayim at the end of the Shema every day. Once again, at first glance our leaving Mitzrayim seems to have no connection to the recitation of the Shema in which we accept the yoke of heaven, קבלת עול מלכות שמים. In fact, they are two entirely separate Mitzvot. However, after further analysis the connection becomes clear. The whole purpose of Yetziat Mitzrayim was our acceptance of Hashem as God, and it therefore fits perfectly into the theme of Shema.
This concept of Yetziat Mitzrayim can also help explain a different section of the Haggadah. In the paragraph of Avadim Hayinu we state that if Hashem wouldn't have taken us out of Mitzrayim we would still be enslaved in Egypt to Paroh. The question raised is that it is impossible to believe that if Hashem did not take us out that we still would have been slaves. At some point over the generations amongst the wars between the different countries we would have been freed from our bondage. However, the Brisker Rav explains that the whole point of Yetziat Mitzrayim was for us to become Avdei Hashem. Even though it is true that if Hashem would not have freed us from Mitzrayim we would have been freed at some later point in time, we would nonetheless stay at the same level and status that we were on when we were slaves in Egypt. It was only as a result of Hashem's taking us out of Mitzrayim that we were able to become Avdei Hashem.