What is the reason for washing the dead before burying them?

When a person dies and is prepared for burial, all of his body is washed in the same way as a living person washes his body to remove the state of ritual impurity. Death marks the end of our life on earth and the beginning of another leading to the hereafter. The departure from this life is thus marked by an act of symbolic purification.

The decomposition that takes place after death is a process which will eventually be reversed when we are resurrected. Therefore, the symbolic gesture of purification is useful because it marks the end of a stage wherein purification is necessary before any act of worship. It signifies that one is approaching the next stage without any lingering impurity.
And God Almighty knows best.