John MacArthur on why doctrine is practical.

Some evangelicals... view doctrine as heady and theoretical, they dismiss it as unimportant, divisive, threatening, or simply impractical.

People often ask why I emphasize doctrine so much. Now and then someone tells me frankly that my preaching needs to be less doctrinal and more practical.

Practical application is vital. I don't want to minimize its importance. But if there is a deficiency in preaching today, it is that there's too much relational, pseudopsychological, and thinly life-related content, and not enough emphasis on sound doctrine.

The distinction between doctrinal and practical truth is artificial; doctrine is practical! In fact, nothing is more practical than sound doctrine.

The pastor who turns away from preaching sound doctrine abdicates the primary responsibility of an elder: "holding fast the faithful word which is in accordance with the teaching, that he will be able both to exhort in sound doctrine and to refute those who contradict" (Titus 1:9). We teach truth, we teach error, or we teach nothing at all.

... No believer can apply truth he doesn't know. Those who don't know the Bible's principles for marriage, divorce, family, childrearing, discipline, money, debt, work, service to Christ, responsibilities to the poor, care of widows, response to governments, eternal rewards, and other teachings will not be able to apply them.

Those who don't know what the Bible teaches about salvation cannot be saved. Those who don't know what the Bible teaches about holiness are incapable of dealing with sin. Thus they are unable to live fully to God's glory and their own blessedness.

http://www.crosswalk.com/spirituallife/11548862/page2/