The seed emotions and thoughts of everything you have ever done are stored within you. These seeds, through the medium of your personal will, gave rise to an action which in turn became the seed for more action. This is the endless cycle of cause and effect, karma. And all this is stored in you. This is the basis for memory and is what is responsible for the habitual patterns of your behavior.
The entire point of spiritual life is to gain access to a new source of light outside of the system of your own karma in order to offset the force of these habitual behavior patterns. The transmission of grace introduces large quantities of light into your system that physically clear the karma from you. This process is called purification.