By bringing all dominance of the flesh to a halt by fasting, a way is made for the Spirit’s direction to be heard and responded to.
Acts 13:2-3 (NIV)
2 While they were worshiping the Lord and fasting, the Holy Spirit said, "Set apart for me Barnabas and Saul for the work to which I have called them."
3 So after they had fasted and prayed, they placed their hands on them and sent them off.
There is direction that comes only after movement in other directions has ceased. However, as momentum increases in a direction already taken, it becomes increasingly challenging to redirect. It is possible to completely turn the wheel of a vehicle that has come to a complete halt, but the more speed is built up, the less possible it is to fully turn the wheel. It will only cause a crash.
As there is a build up of motion that has been directed by the demands of the flesh, there is less and less opportunity for the Spirit’s voice to provide direction. Especially when it comes to being given a whole new direction, until everything else comes to a halt, it might even be a bit dangerous to make those kinds of changes. A voice might be heard, but the chance of that voice being the Spirit’s is very small. There has been so much response to the voice of the flesh, that momentum in its direction will disallow such a redirection.
By bringing all dominance of the flesh to a halt by fasting, a way is made for the Spirit’s direction to be heard and responded to. We have all been called to a work in the kingdom. Discovering that work and moving towards it is greatly helped by purposefully bringing to a halt an appetite for any other than our God.