What was the purpose of Paul's letter before he visited the Corinthian church?

The letter was preparatory to Paul's coming to the congregation in Corinth, so that he could appeal to them in love and meekness rather than coming with a rod, wrath, and anger. It prepared the way for a loving, meek visit instead of harsh measures.

Les explains that in 1 Corinthians 4:21 Paul asks the Corinthians whether he should come with a rod, in wrath and anger and not spare, or in love and the spirit of meekness. Paul would choose love and meekness, because harsh measures were not his nature unless absolutely necessary. Thus this letter was preparatory to his coming to the Corinthian congregation, enabling him to appeal to them in love and meekness.

1 Corinthians 4:21

What will ye? shall I come unto you with a rod, or in love, and in the spirit of meekness?

Why did Paul choose to come to the Corinthians in love and meekness instead of with a rod?

Paul chose love and meekness because harsh measures were not his nature unless absolutely necessary. He wanted the letter to prepare the Corinthian congregation so he could appeal to them in love and meekness rather than come with wrath, anger, and a rod.