Bidding farewell to the dead is a well known custom and it consists of many age old traditions. Both the Christians and the Muslims are buried but pennies are put only at the gravestones of the Christians after burial. The desire to receive worldly goods even after death can be observed in many ancient cultures. For example in the Egyptian culture people used to make high and mighty tombs or the pyramids and fill them up with gold, jewelry and other luxury items so that they can live comfortably after passing away.
You must have noticed how flowers, bouquets, cards and other nice things are placed at the gravestones along with pennies. The first reason is definitely respect and a desire to show that the dead person is still loved, remembered and cherished. Some believe this tradition started with Ben Franklin and people leave pennies on the gravestone for the sake of luck. People can be superstitious and lots of it is associated with death and after life. Another reason for leaving pennies at the gravestone is far more simple and pragmatic. People leave pennies because it is economical and cheaper than putting flowers at the grave.
According to Greek mythology Kharon or Charon is the ferryman who takes the dead souls across the river to the land of Hades, the king of the underworld of the dead. Charon is a demon who serves Hades. There was a custom of putting obolos coins in the mouth of the dead people as the fees for taking the dead person’s soul to the ultimate resting place. Placing the pennies on the gravestone has come down as a tradition to us from the very ancient days so that a dead person’s soul can rest in peace. Without the fees Charon do not take the dead souls to the land of dead. Simple barter logic is working even here.