Why is the idea of "free will" so important to people?
To deny the perfect freedom of the human will is to acknowledge the fallenness and incapacity of humanity. People love to hear that the human race is "basically good" and "full of potential," and that if you would simply follow your dreams faithfully, you can be or do whatever you want. We hear these lies all the time! Our minds are saturated with the elevation (worship) of human free will.
In the King James version, one of the things Paul says in Colossians 2:23 about false religion, he calls it "will worship." This captures the essence of where all false religion ultimately points—the worship of the human will, the worship of self.
The heretics Pelagius, Joseph Arminius, and Charles Finney all taught in one way or another that you do not need God for your salvation, your sanctification, or for anything else. You have the power to effect it in and of yourself. So when the Bible commands you to "repent and believe," despite it also stating that man is unable to do so, you have the power to repent and believe in and of yourself. And when the Bible commands you to "Be holy, for I am holy" and to "be perfect, as your heavenly Father is perfect," you do not need Jesus to help you because you have the power to be holy and perfect in and of yourself.
Why is the idea of "free will" so important to people? Because they do not want to admit the depravity of their own heart. Though they know the thoughts and intents that flow through their mind on a daily basis, and how wicked they know themselves to be, nevertheless they want to attempt to deny this and pretend that it does not exist. They want to believe themselves "basically good," despite their thought life convicting them otherwise. The idea of "free will" puts man in charge and does what man has always desired—to worship self.
Christians who believe in the idea of "free will" do so at the expense of the words of Jesus and the truth of God's Word. Jesus Himself made it clear that "No one can come to Me unless the Father who sent Me draws him" (John 6:44), and that "no one can come to Me unless it has been granted him from the Father" (John 6:65). Romans 6 makes it clear that man's will is slave to sin and the devil. John 3 makes it clear that you need to be born again from above, which eliminates any participation on man's part in being regenerated and born again spiritually. Not only that, but John 1:12-13 also makes it clear that you do not become a child of God by (1) natural descent, (2) human decision [i.e., the human will], or (3) human declaration. Likewise, Romans 9:16 makes it clear that it does not depend on (1) man's will, or (2) man's efforts. Romans 9:19-23 ought to be a sobering realization to anyone who truly loves God.
Hebrews 12:2 states that Jesus is the author (initiator) and finisher (perfecter) of our faith. Philippians 1:6 states that what God begins in us He will finish by bringing to completion. There are literally dozens upon dozens of passages in Scripture that the Arminianist (the Christian who advocates the idea of "free will") must ignore and/or discard in order to argue for the concept of "free will." Why do they do this? Because they want man to be at the helm; they want man to be in charge. When you begin to accept the truth that all humanity deserves to spend eternity in Hell, and that without the grace of God choosing to save you for no other reason than God's glory, then you will worship God like you have never done so before. There was nothing in you that merited salvation. You did not deserve it. It was God's goodness, grace, and pleasure that chose to save you for no other reason than God's glory. Apart from Jesus, you are nothing!
"By grace you have been saved" (Eph. 2:5, 8), "not of yourselves" (v.8).
ADDENDUM:
It actually exists!!! Here is a set of highlighters. Do you notice the black one on the right? That's called a (hide)lighter. It covers up text, though still remaining legible. After copying the page, it becomes illegible. This is the Arminian's tool of trade for reading and studying the Bible. In fact, it is the tool of trade for every denomination and every Christian who does not strictly adhere to the Word of God as their sole and final authority on all matters pertaining to doctrine, congregational practice, family life, and personal holiness. If it disagrees with their theology, they just (hide)light it. *POOF!* No more pesky verses that contradict their position.