The grave includes only the decayed corpse, while the soul had departed from the body by the death.
Man is comprised of a material body and an ethereal or spiritual soul.
Most people think that the dead man is in his grave; so they go to visit him: in case he is one of their family members or a dear friend, and they speak to him as if he is in his grave, and some, specially the Western peoples, put some flowers on the grave.
The ancient had such idea that the dead is in his grave, so they put with him in the grave his wealth: of gold and silver ..etc.
But some spirits are imprisoned in the graves following the death.
Quran 35: 22
{God makes to hear whomsoever He will h, but you cannot make those in the graves to hear i.}
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h i.e. He guides, to the Islam religion, the one who is worthy of the guidance.
i God – be glorified – likened the stubborn associaters to the spirits imprisoned in the graves; as such are the associaters bound with the traditions of their fathers.