The rod plane is a style choice, but it decides which water you can fish. The guide calls horizontal false casting ‘a great fishing skill’ and teaches casting a tight loop under a low-hanging obstacle with the Horizontal Casting Drill.

What follows from those rules:

• Cover. The line goes where the rod plane points, so a horizontal plane carries a tight loop under overhanging branches and bankside brush that an overhead cast would hit.
• Wind. With the wind blowing onto your casting arm side, an off-side plane tilted away from the wind can improve safety, and Task 22 names a horizontal cast among the safe methods for that condition. This is a condition, not a general rule: a horizontal plane is not automatically safe in every wind direction, so read the wind before you flatten the plane.
• Options. The guide tells candidates to learn these tasks to develop the ability to cast in multiple planes, and to match casting style to where and how a student fishes — sidearm for saltwater, vertical in small streams.

Learning several planes lets you pick the plane the water needs.

Source

From the CI Study Guide (2025) (PDF), Task 7 (rod planes), the Horizontal Casting Drill (Appendix A) and the wind task (Task 22); terms from the FFI Fly Casting Definitions (Rod Plane).