A) Because in the Bible a mark on the forehead is a symbol of a person’s ownership. By having their foreheads marked with the sign of the cross, this symbolizes that the person belongs to Jesus Christ, who died on a Cross. This is the imitation of the spiritual mark or seal that is put on a Christian in baptism, when he is delivered from sin.
B) Because ashes are a biblical symbol of mourning and penance. In Biblical times, the custom was to fast, wear a sack cloth, sit in dust and ashes, and put on dust and ashes on one’s head. While we no longer normally wear sackcloth or sit in dust and ashes, the customs of fasting and putting on ashes on one’s forehead as a sign of mourning and penance have survived to this day. These are the two key distinctive of Lent. In fact, Ash Wednesday is the day not only for putting ashes on one’s head, but also a day of fasting.