Grape stomping is a custom associated with the harvest, going back to Dionysus, the Greek god of wine.
Dionysus’ life is symbolic of the vine’s life cycle. As legend goes, Dionysus was dismembered by Titans and returned to life, signifying vine pruning at season’s end, dormancy to rejuvenate in winter and rebirth in spring.
Festivals honoring Dionysus still exist, with pigeage (French for grape stomping) central to those rituals. Stomping is symbolic of dismembering vines (just as the Titans did to Dionysus) by crushing grapes (breaking the flesh) and releasing the juices.
Grape stomping is rarely practiced today, though many wineries sponsor grape stomping festivals, with proceeds often donated to charity (the juice is either discarded or used as fertilizer).
That said, Portugal still uses stomped grapes in some port wines. And the practice also continues in a few small wineries in Spain and in Cinque Terre, Italy.