According to the concepts of the soul, the soul of a  dead person is supposed to stay at a spirit tablet (or ancestral tablet) after the dead body was buried.

       The sedan seat can be the seat of the dutiful eldest son who carried the spirit tablet, or the spirit itself.

       It won't be necessary for the eldest to sit on the sedan.  If he carries the spirit tablet home, maybe it’s a good way to express his sincere.  That’s why the spirit sedan is not absolutely necessary.