The point in showing it once is so we can get a backstory. Let me try to explain.

Shortly after we see the horn, we get a backstory involving the Hozuki. As it turns out, at one point in time, Shinkuuro wanted this monstrosity put inside him. He was eager to earn Benika's respect. But I surmise he's regretted it. To him the horn is artificial strength. And for viewer purposes it gives an insight on Shinkuuro and his non-static personality.