1. The Gemara explains that the ashes are placed on the same place on the head as the Tefilin are worn.
2. The Gemara cites two reasons for why the public goes out to the main street of the town to pray on the last seven fast days.
3. We put ashes on the Aron to indicate that Hashem is suffering along with us.
4. There is a dispute about why the ashes are placed on each man's head.
5. There is a similar dispute about why they also would go to the cemetery.
A BIT MORE
1. The verse says, "Glory will be put on the heads of the mourners of Zion instead of ashes." This implies that the ashes should be placed on the same place on the head as the "glory," which refers to Tefilin.
2. Rebbi Chiya bar Aba: We have cried out in more private areas and have not been answered, and therefore we will now denigrate ourselves in public with the hope to be answered. Reish Lakish: This is similar to exile, and it should atone for us.
3. The verse states, "I am with him in pain," and, "In all of their difficulties, it is difficult for Him."
4. Some say that it is a humbling gesture, showing that we consider ourselves before Hashem like ashes. Others say that it is to remind Hashem of the ashes of Yitzchak (a reference to the ram that was sacrificed instead of Yitzchak).
5. Some say that it is a humbling gesture to say that we are considered as the dead. Others say that it is to cause the righteous dead people to pray for us.