SIGNIFICANCE OF SHRADH
The soul incarnates into a physical body ...
The soul incarnates into a physical body which is made up of the five elements in various permutations and combinations, reincarnating birth after birth depending on the kind of karmas which one does in a lifetime. There are certain souls who do not reincarnate, either because they are finished with their karmic cycle of bhu lok [the prithvi lok] or because they are stuck in a different dimension of existence and are unable to manifest in the human body, as their karmic balance is not sufficient to entitle them for a human birth. These souls remain suspended as energies and are called ghosts and spirits or they go to a different dimension of existence called the pitr lok or the lok of the ancestors.
Whatever dimension or level of existence that they are in, unless they have been freed from the cycle of birth and death, they need energy from their off-springs to offset their karmic balance to enable them to reincarnate again.