Paul tells Timothy Scripture is useful for teaching. We don't use it to teach math. We don't use it to teach physics or geography. We don't use it to teach electronics or computer science. We would judge the teacher who used it for these subjects to be a very poor teacher indeed.
Scripture is useful for teaching doctrine.
The most important doctrines, of course, concern God, Jesus Christ, and our salvation. Correct doctrine is important, very important. Without the benefit of correct doctrine, without the benefit of truthful teaching, God's people stray from the truth and fall into falsehood. Only a thorough knowledge of and grounding in Scripture can keep us from being destroyed. When a new FBI agent is hired he is given intensive training.