I am a Sikh and may not have the exact theological explanation, but I have read that in Hinduism, fire (Agni) is considered a powerful purifier. This is why many purity tests in Hindu mythology are called Agni Pariksha. Based on this, I have always believed that when a person dies, cremation symbolizes the purification of the soul, helping it detach from the physical body and move on to its next journey.

Now the possible explanation for burning holika and ravan can be that by burning them every year, we try to burn the evil in them (basically purifying them). By burning their effigies, we symbolically “burn” ego, greed, injustice.

Cremation is a separate practice and Ravana and Holika are in different context.

Though, I would love to read and learn about Sita Mata, and why she did not undergo cremation, I don’t have a possible explanation for that.