Why did paul write to the galations?

It was Jehovah God who used His spirit to tell Paul what to write to the newly formed congregation in Galatia. (2 Peter 1:20-21) He saw to it to emphasize that there were those that would try to mislead the Christians with false information, and that they would have to reject the falsehoods in favor of the truths the Christ had taught. (Galatians 1:6-9)

Paul also reminded them that it was Jehovah that had chosen him to be a disciple of Christ. (Galatians 1:15-17) He gave an example of the importance of avoiding prejudice, especially amidst fellow believers, by recounting how he had to correct the apostle Peter for such behavior. (Galatians 2:11-14) He brought out that the Mosaic Law being followed was no longer important, but having faith in the Christ and showing it through action. (Galatians 2:15-16; 3:2, 5, 6, 10-14) He let them know that there would be a war within themselves between the flesh and the spirit, a.k.a. between doing what was right in Jehovah's eyes and what wasn't. (Galatians 5:13-26) Paul also brought out that a person will reap what they sow, whether for better or worse. (Galatians 6:7)

(New World Translation of the Holy Scriptures)