1. Anything that has been in a person for a long time has developed roots. 2. Anything that has roots will become a stronghold. Strongholds require power to be demolished, they can not be destroyed by a simple prayer, they require prayer and fasting.
The main purpose of fasting is to strengthen our relationship with God. When you fast, it sharpens your spiritual ear. In the bible we see how Daniel fasted and received dreams and visions from the Lord.