Why? Mostly because it captures in lyrical form God's irresistible grace, the concept of Reformed soteriology that if God has called you to salvation, then he will send his Holy Spirit to change your heart until you come to believe in Jesus and surrender your life to him.
It also debunks the false notion that God will not tamper with your free will. I tell you, I'm glad that he very much does that, and here's the lyric from "Reckless Love" that backs it up:
"There's no shadow You won't light up
Mountain You won't climb up
Coming after me
There's no wall You won't kick down
Lie You won't tear down
Coming after me"
and you also have the verses that say, "Before I spoke a word, you were singing over me." "Before I took a breath, you breathed your life in me."
My beliefs aren't really popular. One, people love their free wills and protest, "So we don't have a choice in our salvation?" I say that people have wills and will follow them, but our wills aren't free. Without God's intervention, we can choose God, but we won't because we love ourselves and our ways so much more. Our hearts need to be changed by none other than the Holy Spirit. It's like and illustration I've used before of me going to a graveyard and inviting people there to a party. They have the choice to go, but they can't make that choice - they're dead. They must be made alive.
The other objection is the idea that not all people will be saved. What about 2 Peter 3:9 which says, "The Lord is not slow to fulfill his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance"?
I honestly believe Peter is directing that sentence to all people that God has called to salvation, the elect. He will not will any of them to perish, and they won't perish. But if God means that he wills all people everywhere to be saved, then everyone will be saved. But not everyone will be saved, so he must just mean that God doesn't will his elect to perish. This idea is backed by Jesus's prayer in John 17:9, "I am praying for them. I am not praying for the world but for those whom you have given me."
God wants us to share the Gospel of Jesus to all people, but only those he has called will respond and believe, and those are the people he will chase down recklessly until they turn to him. And he will wait until the last one of them repents before he finally sends Jesus back and ends this current age.
And if you are one of the believers, you have a lot to be thankful for. You should not be proud or complacent. You must be humble knowing that God's grace is not obligated, and you must not be complacent because you must still work your salvation with fear and trembling. You must spread the Gospel to all people and you must work for justice and you must watch yourself so that you don't sin, but you go knowing you won't be alone and that you will see the fruit of your labors because God plans the means as well as the ends. The idea of a universe where something is outside of God's control is not possible because God must be absolutely sovereign, even when we don't understand it or are comfortable with it. But he's also good and loves his creation more than we ever could. These things we will never quite understand because we are not him. And for that I am glad.