They didn’t have to, but if there corpse was laid to rest with coins over there eyes, or if they were cremated with some form of wealth, then they would use this to pay Charon to cross into the underworld, those who could not pay could not cross and so could not receive their afterlife and were left in a sort of limbo. Even if they somehow made it across the river without aide of the ferry man, then Cerberus would stop them going any further.