In an article I found online, the following gave me pause:
As our 'ye'ud' - our destiny - is to become a nation that will serve Him, G-d found it necessary to send us down to Egypt in order that He could redeem us. This could be the deeper meaning of Rashi's interpretation of the pasuk "ve-higgadeta le-bincha ... ba'avur zeh" - that we must explain to our children that G-d took us of Egypt in order that we keep His mitzvot. [See Rashi & Ibn Ezra 13:8.] Rashi understands that the primary purpose of "magid" is not simply to explain why we are eating matza, but rather to explain to our children why G-d took us out of Egypt - or in essence, why He has chosen us to become His nation and hence keep His mitzvot. To complement this thought, we will show how this same theme may relate as well to the very purpose of G-ds first covenant with Avraham Avinu - "brit bein ha'btarim".
Why were our slavery to Pharoah in Egypt and subsequent Exodus necessary in order to become a nation that would serve G-d? Could this goal not have been achieved by Avraham and his descendants without slavery and Exodus?
Additionally, why did Avraham need to be told about the forthcoming slavery and Exodus?