Paul the apostle wrote this second letter to Timothy, a close friend and travelling companion. Timothy had accompanied Paul on part of his journey as a missionary. Timothy was a young leader in the church. Because of his age, he may not have gotten much respect. Paul was writing to Timothy to encourage and to guide him.
The immediate context of this passage is proper living. Prior to his statement on Scripture, Paul told Timothy about a time when people would act corruptly. Paul then contrasts Timothy with those people. He encourages Timothy by reminding Timothy of his background in understanding the Scriptures. Paul reminds Timothy that all Scripture is breathed out by God. He does this right before informing Timothy of four uses for Scripture.
The first use for Scripture that Paul mentions here is teaching. As Christians, we are to learn Scripture and do our best to understand it. This is vital if we are to live lives that effectively lead people to Jesus Christ. A teacher of the Word must teach to the best of his or her ability, and a student of the Word must learn to the best of his or her ability.
The second use for Scripture that Paul mentions here is rebuking. To rebuke someone is to tell them that they are wrong. In other words, one of the uses of Scripture is to point out the wrong. This may not be the most popular thing to do, but it is necessary at times. Whenever a false doctrine or teaching comes out, it is important that teachers of the Word nip it in the bud before that doctrine takes root.
The third use for Scripture that Paul mentions here is correcting. Whenever someone sins, Scripture can be used to help correct the issue at hand and lead a straying person to God, or back to God. Scripture can also be used to correct false doctrines that may have already started to take root. Scripture can also be used to correct a group of people. Scripture presents truth. Because of this, Scripture can be used to correct a lie that a believer may have ended up believing.
The fourth and final use for Scripture that Paul mentions here is training in righteousness. Scripture can be used to train those who are already believers in Jesus Christ in living the Christian life. It can be used to help teach us how to be more effective leaders, teachers, and evangelists. Without guidance, we are like a boat with no motor or paddle--we have no idea where we are headed, and no way to help us get to the place where we need to be. This is why Scripture reading is vital to the Christian. The Bible is an instrument that God uses to train us, teach us, equip us, and guide us. It is important that we always remember that.