Why was it an encouragement to Paul that he did not have to go to Corinth?

It encouraged Paul because the Corinthians had apparently corrected their problems, and his first letter had accomplished its work. They had dealt with the man involved in gross immorality and restored him to fellowship, so Paul did not need to make a personal visit to confront them.

Les explains that II Corinthians was a follow-up to I Corinthians, where Paul had to deal with many problems in the Corinthian church—more than in any other congregation under his apostleship. In the first letter he had to be hard with them, but in the second letter he defends his apostleship and encourages those who had seemingly corrected their problems. Les points out that this itself was an encouragement to Paul because he had intended to go to Corinth personally, but word came that his first letter had accomplished its work. The Corinthians had dealt with the person involved in gross immorality, and he was evidently back in fellowship, so Paul did not have to go personally and come down on them.

2 Corinthians 1:15

And in this confidence I was minded to come unto you before, that ye might have a second benefit;

What did Paul mean when he said he wanted the Corinthians to have a second benefit?

Paul meant he wanted to visit Corinth personally and deal with the church's problems face to face, so they would receive a second benefit. He intended to pass through Macedonia and return to Corinth, but he did not make that trip.