Fasting is a way to discipline the flesh and attune to the Spirit.
Fasting helps us overcome certain habits we are prone to viz anger, profanity, gluttony, etc
It also helps build our prayer life.

It is a way to express our desire (desperation) for more of God as we sacrifice our fleshy yearnings, especially food, sex, money and sleep.

The most important reason for fasting is to increase our Faith.
There are certain strong holds, doubts and unbeliefs in us that only through fasting can we drive them out.

Matthew 17:19-21
Then the disciples came to Jesus privately and said, “Why could we not cast it out?”
So Jesus said to them, “Because of your *unbelief;* for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you. However, this kind does not go out except by prayer and fasting.”

Jesus was not addressing driving out the demon but driving out their unbelief.
Demons are not a problem to the Holy Spirit. It is our Unbelief that limits the Manifestation of the Holy Spirit.

Fasting goes together with studying the Word and praying in Spirit.
We should try not to have our own personal desires when we want to go into fasting.
We should empty ourselves in order to hear clearly what God is saying; whether or not it pertains to our own desires.
Truly, the answer to our own desires will be directly or indirectly connected to the prayers and revelations we will receive from God.

May the Grace for prayer and supplication be multiplied to us all.