Most religious traditions require a blessing before we eat; Judaism adds the obligation of blessing after we eat. This prescription is based on a Biblical passage from the book of Deuteronomy:"When you have eaten your fill , give thanks to the Lord your God." More literally the phrase is translated as :"You shall eat, be satisfied and bless your God."
It is a commonplace that once we have eaten and feel satiated, the desire to think of the source of our satisfaction becomes somewhat distant. We are not inclined to think of spiritual nourishment at that time. When the belly is full, our souls seem to slumber.For this reason, our tradition enjoins us to awaken our souls "to give them an opportunity to express their yearnings, their cravings for God."( Breslov Haggadah)
On the other hand, perhaps the Biblical reference to "You shall be satisfied" points to the feeling of gratefulness as a prologue to blessing. When we feel grateful ie. satiated, fulfilled, we are then spiritually inclined to acknowledge the ultimate source of this feeling of well-being, and to bless God.
At this point in the Seder, we have engaged in many steps on our journey toward spiritual freedom. We have recited the story, completed the rituals, eaten to satiety a festive repast. All that is left to do is to respond to God out of a deep feeling of gratitude. Thus we bless; beyond blessing is Hallel-pure, and unvarnished praise , the next step in our journey.