If we believe the Bible to be the infallible Word of God, why don't we implement it in our defense of the faith? Apologists for the last two hundred years have focused on defending the Christian faith using human reason, physical evidence and proof, arguments for the existence of God, etc. But that's not what Jesus calls us to do: rather, He calls us to rely on His authority to defend the truth He has delivered once for all to the saints.
When we present arguments and evidence based on reason and divorced from the authority of Scripture, we're telling our audience that they're the final arbiters of truth rather than telling them (as we ought) that they must submit to the authority of Christ as revealed by Him in the Bible. In other words, it isn't the unbeliever's job to decide whether Christianity is true or not, it's the Christian's job to tell unbelievers that they must submit to Christ's authority or be cut off from Him forever.
It's not suprising, given the history of Western thought, that apologetics has taken the turn it has. Modern Enlightenment philosophers and their heirs have affirmed man's intellect as the sole arbiter of truth, meaning that anything that is true must be able to be empirically verified and apprehendable by the mind of humans. Christians, instead of reaffirming the primacy of God's Word and the final authority of Christ, have been quick to submit to these humanistic standards, and have tried to commend the faith to unbelievers using the standards established by those who reject God.
And unbelievers do reject God. There isn't a person who can say they don't believe in God because there isn't enough evidence: the Bible says that all people know God, because they have been created by Him and in His image, and that they suppress this truth in unrighteousness. This means that so-called "unbelievers" sin in order to forget about the God they know and to whom they know ultimately that they're responsible.
Our job as Christians is not to approach these unbelievers timidly, offering proofs by which they can determine Christianity is real and thereby put their faith in Jesus. Rather, we are to preach the Gospel to them, and when opposed we're to point them back to the Word of God where they'll find their sins condemned and the free offer of God's graceful forgiveness clearly presented to all those who are in Adam, our first father and the one through whom sin entered the world.
Presuppositional apologetics is designed to equip Christians to defend their faith using the Bible as the foundation for everything they believe, as well as their defense of that belief. Wait, some of you will say, isn't that circular reasoning? If we use the Bible to defend the Bible, haven't we gone in a self-defeating loop? To which we'd respond: the difference between the presuppositional argument and the unbelieving argument is that we have a foundation for our circular argument, and unbelievers do not.
There's a sense in which all arguments are circular. When someone says, "I only believe what I can observe and understand," they're making a circular statement. If you ask them in response, "On what do you base that assertion," what can they say except that they base it on observation and their own understanding? The difference between those types of answers and the presuppositional response to accusations of circularity is that presuppositionalists actually have a foundation for their argument.
That foundation is the Bible. The Bible, for Christians, isn't just a book. Instead, it's the written self-revelation of God, telling us where we came from, explaining our predicament, presenting God Himself, and laying out the salvation extended to all those who believe in Jesus Christ for the forgiveness of their sins. The Bible is God's Word, and therefore it is sufficient in itself to defend the claims that it makes.
Presuppositionalism is sometimes seen as difficult, and while some of its chief exponents' work can make for difficult reading at times, in the end it's quite simple and can be employed by all Christians. The presupposition to which its name refers is that God exists and can be known. This is the basis for the good news of Jesus Christ that we are to take to every tribe and nation, and the only true basis for any defense of the hope we have and the faith delivered once for all to the saints.