“Why? If the soul has left, isnt it a useless shell?”
Veneration of relics (most often body parts) of saints is nearly as old as The Church. If I recall correctly, and Alex can if I am wrong, such veneration began in 155 AD after the martyrdom of +Polycarp.
It’s funny you mention St. Polycarp, since today was his feast day on the pre-Vatican II sanctoral calendar. :-)