Gratitude to God is the most dominant motif of the Jewish liturgy. It is expressed through shevach, songs of praise to God. Its most exultant forms are the Hallel (Psalms 113-118) and Hallel HaGadol (Psalm 136). The latter shevach was chanted publicly in the Temple with its rhythmic response "For His lovingkindness endures forever," ki l'olam chasdo.
Hallel is recited on Festival days, but it was omitted from the Purim liturgy because, as Rav Nachman explained: "Its reading (i.e. the Megillah) fulfills the requirement to recite Hallel." (TB Megillah 14) Shevach is implicit in the very recital of the Purim narrative and no additional Hallel was necessary. If so, why was Hallel included in the Haggadah when the narrative of the Exodus itself should have satisfied the shevach requirement?
It seems that shevach is not enough for the Seder night. We are expected to rise to higher levels of exultant praise, to a shirah chadasha. It is a night when the Jew is in love with God, a night of passionate romance which is reflected in the tradition of reading Song of Songs after the Haggadah. We move from Hallel HaGadol to Nishmat kol chai, "the breath of every living being shall bless your name." We ecstatically see all of creation joining in a grand symphony of homage to God for all the blessings of life. From the geulat Mitzrayim we are gripped with an appreciation that God is also the ultimate salvation of all mankind. The concluding note of the Haggadah is an eschatological vision of a glorious future, when "every mouth shall give thanks and every tongue shall swear allegiance unto You; every knee shall bow to You." To highlight this added dimension of gratitude, the Hallel and the Hallel HaGadol were included in the Haggadah.
- Rabbi Joseph B. Soloveitchik, Reflections of the Rav (pp. 215-216)