That's because the pitcher is the ace of the team. If he simply batted, he wouldn't be a good main character, mainly because he would be sitting down or just standing around 90 percent of the time.

Seriously though. even if he is great at batting, that doesn't mean that he is less talented in pitching. What was hinted at with his batting skill is his athletic ability and baseball sense. It just so happened that he turned out to be an even better pitcher than he is a slugger.

Wakaba didn't tell him directly, but she did tell the thug guy about her dream (the sleeping sort). Koh was the pitcher and the thug was the catcher at Koshien. Koh was made aware of this a few episodes ago. Wakaba also said (to Aoba I think) before that 160km/h, and being the best pitcher in Japan is possible for Koh. I think that is more than enough reason for him to be a pitcher.