Both Demeter and Dionysus are what are sometimes known as "vegetative" or "fertility" deities, connected with the harvest and fertility in all aspects. Demeter in particular is a goddess of corn (used in its generic sense of grain, not the United States term referring solely to maize) and is particularly important as a goddess of crops and harvest. This connection is visible in the withering of the crops when Demeter mourns the loss of Persephone. The Eleusian mysteries celebrate Demeter in her roles as a fertility goddess. Dionysus is the god of wine and drunkenness, and thus concerned with the grape harvest and wine making. His rituals and cult, as portrayed in Euripides' Bacchae, express a more orgiastic, and perhaps eastern aspect of fertility.
Demeter (greek) or Ceres (roman) is goddess of agriculture, crops, barley etc. So she is basically all the harvest food. Dionysus (greek) or Bacchyus (roman) is god of grapes, wine and festivals, so he is the drink god. Demeter provides the food and Dionysus provides the drinks. Demeter is more useful since she is actually goddess of the harvest but he is also part of the harvest, being god of wine and grapes.
Hope this answers your question. P.s This is what I think.