The Talmud records a debate from ancient times in which the foremost teachers and interpreters of the law debated how to eat the seder's required foods: meat from the Passover sacrifice, maror, and matzah.  Many rabbis wrote that each food should be eaten separately.  They said each food has its own blessing and its own symbolism. Eating them together would be unnecessary and even confusing. Rabbi Hillel, however, argued that by combining the symbol of slavery (maror), the symbol of redemption (matzah), and the symbol of God's covenant with the Israelites (the Passover Offering), we remind ourselves that even the most bitter circumstances must end.

Because the Temple is no longer standing, there is nowhere to offer the Passover sacrifice, so charoset is eaten in the Hillel sandwich rather than the sacrificial lamb. 

Eat a sandwich of bitter herbs, charoset, and matzah as we read together:

In remembrance of the holy Temple, we do as Hillel did in Temple times.  He put matzah and bitter herbs together and ate them as a sandwich with the Passover Offering, in order to observe literally the worst of the Torah, "They shall eat [the offering] with matzah and bitter herbs."

haggadah Section: Koreich
Source: Adapted from Ariana Silverman