If the practical Mitzvot are so significant, one might ask why the Patriarchs served G-d primarily in spiritual ways? The answer is that the Patriarchs lived before the Jewish people had been enslaved in Egypt.
In the time of the Patriarchs, the physical world was too coarse to be able to express G-dliness through the practical Mitzvot of the Torah. Hence their service had to be primarily through their spiritual qualities of Love, Awe and Truth. But when the Jewish people entered Egypt, the most immoral and materialist land in the ancient world, and were then enslaved there, and were finally set free by G-d acting through Moses – they purified the world as a whole.
After that they could receive the Torah at Sinai, which teaches how G-dliness can be expressed through the physical aspects of life. As a result, we the Jewish people now, are able to serve G-d thought the various kinds of practical Mitzvah, and attain through them an exalted level of self-transcendence, which indeed bonds us to G-d in a wonderful way.
We now understand why the answer to the question includes mention of the slavery. It is the slavery and the Exodus which brought us to the next stage of history, in the which through the physical Mitzvot we can achieve the highest levels of self-transcendence and bonding with the Divine.
