What importance does dandelion wine play in the book Dandelion Wine?

Answer 1

It's symbolic of life, the summer and the joy of being alive:

"In the summer of 1928 in Green Town, Illinois, 12-year-old Douglas Spaulding realizes he is alive - that is, he becomes self-aware in a manner he had never been before, and is exhilarated by the new richness it gives everyday experiences, especially at the start of summer: the harvest of fox grapes, the making of dandelion wine, the purchase of new sneakers, the setting up of the porch swing."

or, maybe even better:

"The title refers to a wine made with dandelion petals and other ingredients, commonly citrus fruit. In the story, dandelion wine, as made by the protagonist's grandfather, serves as a metaphor for packing all of the joys of summer into a single bottle."

Answer 2

The dandelion wine is brewed in the summer. Drinking it in the winter evokes the summer. "Dandelion Wine" is Bradbury's evocation of his own childhood.