Why did Paul write his letters to the churches?

The letters to the churches were written because the churches had already been established through Paul's ministry, and they now needed doctrine, correction, and instruction. The Holy Spirit prompted the Apostle to write these various letters so the churches could receive the teaching and guidance they required.

Les explains that after Israel would not believe and receive her King, God raised up the Apostle Paul and made known that He was sending him to the Gentiles, opening the Age of Grace and the Church Age. Paul established churches throughout Asia Minor, Greece, and Antioch in Syria from about 40 A.D. until he became a prisoner in Rome around 65. From about 58-59 he began writing his letters. The churches had been established and now needed doctrine, correction, and instruction. The Holy Spirit was behind it all, prompting the Apostle to write these various letters. Paul had already written to the Romans, and the Corinthian letter is now being studied.

Why did Paul start writing letters to the churches he established?

After establishing churches throughout Asia Minor, Greece, and Antioch in Syria, Paul began writing letters around A.D. 58–59. The churches needed doctrine, correction, and instruction, and the Holy Spirit prompted him to write these various letters. These letters addressed the needs of congregations already founded during his missionary work.