Vajrasattva practice is fundamentally about purification. He is a purification deity.
Through this practice, we purify the obstacles and difficulties that arise from negative karma or from broken samaya. These are what we cleanse.
Once we are able to purify negative karma and the consequences of broken samaya vows, we can access the wisdom mind through the innate purity of our own mind.
The defining quality of Vajrasattva is his purifying function.