Zoro was a swordsman, not a hand-to-hand combat fighter.

While he knew how to throw a punch or two, which were very basic, that's where his knowledge ended. He didn't thought he'll need it, if he were to be honest.

Unfortunately, sometimes, he had to fight opponents who fought with their bare hands, and it became a bit of a problem. His forte was swordsmanship, not fights that required much physical contact.

So, he figured he'll learn a little more. And, despite his bashfulness, he approached you for help.

Learning a bit more wouldn't be a bad idea after all, it was for the benefit of defeating more opponents.