How has having a teacher impacted your Zen practice?

I'm aware that Zen places a strong emphasis on the teacher-student relationship, and I'm curious to hear from practitioners: what has working with a teacher brought to your practice that you couldn’t have found on your own? I know that a teacher or Roshi is necessary for koan study, but I’m especially interested in other kinds of challenges—those moments in meditation when you needed personal guidance that a book or video couldn’t provide. I'd love to hear your personal experiences with this.