Sackcloth and ashes were outward signs that the Hebrew people used as an outward sign of one’s inward condition. Such a symbol made one’s change of heart visible and demonstrated the sincerity of one’s grief and/or repentance. It was not the act of putting on sackcloth and ashes itself that moved God to relent and forgive the people of Nineveh in today's reading from the book of Jonah, but the humility that such an action demonstrated.
God sees the heart. God knows the truth. Perhaps this is the very reason Pope Francis, when asked if certain people were sinners, responded "Who am I to judge?"
I wore ashes on Ash Wednesday all day long to witness externally that I would take Lent seriously and make sincere efforts to return to the Lord. The rest of Lent, no one sees my efforts, except the Lord. What does God see in my heart today?