Why did St. Paul have to write and explain the he wanted Christians to be united?

Yes he did and here is why. Paul spent his life teaching Gentiles that they could be Christians without becoming proselytes (converts). This was very displeasing to Jews generally, for they thought of the Mosaic Law as binding for all people, and they were bitterly prejudiced against any uncircumcised Gentiles who presumed to call themselves disciples of the Jewish Messiah. While Paul taught Gentile Christians to stand like a rock for their liberty in Christ, he did not want them to be prejudiced against their Jewish fellow Christians but to regard them as brothers in Christ. Paul did not want to see two churches, a Jewish church and a Gentiles church, but one church: Jews and Gentiles in Christ.