Below is a small excerpt from my senior thesis on burial practices in antiquity:
The practice of burial in the Roman world was rooted in the idea that death was merely a prolongation of life spent under the earth. There is really no dichotomy between burial and cremation as all cremations inevitably involved some sort of inhumation. For the Greeks, after a body was burned upon a pyre, the ashes were buried in the ground. For the Romans, a cremated individual was only humatus at the point when a portion of the body, often the finger, was cut off and buried. A family was free from legal defilement only after this had been done. After the burial, the family would return to the house to cleanse the house of the ritual defilement with the feriae denicales.
Whereas inhumation originated in the belief that the soul of the individual would spend the rest of their life under the earth, cremation seems to have come out of the idea that a body and soul should be separated violently and permanently. Quoting Plato’s Phaedo, Guthrie explains that Plato believed souls were enslaved as ghosts by the fact that they had “become contaiminated with the earthly and the corporeal” thus the cremation served as a final purgation of the corporeal from the soul. Servius, seemingly influenced by the Neo-Platonic philosophy of the 4th century, writes that the body is burned to allow the soul (anima) to return from the principle from which it originated. E. Rohde posits that the burning of the body was to finalize the separation of the deceased from the land of the living. It both freed the spirit, allowing the soul to find rest, and served as an apotropaic rite, keeping the spirit away from the living. Interestingly, Guthrie records a story of Melissa, the deceased wife of Periander, coming to him and telling him that she is cold because the clothes which had been buried with her were not burned, thus they were unable to accompany her in the spirit world because they did not have “spirit-existence.”
 Cicero, Tusculanae Disputationes I, 16, 36
 Rush, 241
 Varro, De Lingua Latina 5.23
 Toynbee, 50
 W.K.C. Guthrie, The Greeks and Their Gods (Boston: Beacon Press, 1950), 274-275.
 Servius, Commentarii in Virgilii Aeneidem III, 68 – …ut statim anima in generalitatem, id est, in suam naturam rediret.
 Rohde, 19-21
 Guthrie, 275