Function in FallbackController generated by Phoenix generator is never called. What is it for?

I used mix phx.gen.json to generate a simple User resource. Among the files it generated, two of them are

user_controller.ex

defmodule MyapiWeb.UserController do
  use MyapiWeb, :controller

  alias Myapi.Accounts
  alias Myapi.Accounts.User

  action_fallback MyapiWeb.FallbackController

  def index(conn, _params) do
    users = Accounts.list_users()
    render(conn, "index.json", users: users)
  end

  def create(conn, %{"user" => user_params}) do
    with {:ok, %User{} = user} <- Accounts.create_user(user_params) do
      conn
      |> put_status(:created)
      |> put_resp_header("location", Routes.user_path(conn, :show, user))
      |> render("show.json", user: user)
    end
  end

  def show(conn, %{"id" => id}) do
    user = Accounts.get_user!(id)
    render(conn, "show.json", user: user)
  end

  def update(conn, %{"id" => id, "user" => user_params}) do
    user = Accounts.get_user!(id)

    with {:ok, %User{} = user} <- Accounts.update_user(user, user_params) do
      render(conn, "show.json", user: user)
    end
  end

  def delete(conn, %{"id" => id}) do
    user = Accounts.get_user!(id)

    with {:ok, %User{}} <- Accounts.delete_user(user) do
      send_resp(conn, :no_content, "")
    end
  end
end

and fallback_controller.ex

defmodule MyapiWeb.FallbackController do
  @moduledoc """
  Translates controller action results into valid `Plug.Conn` responses.

  See `Phoenix.Controller.action_fallback/1` for more details.
  """
  use MyapiWeb, :controller

  def call(conn, {:error, %Ecto.Changeset{} = changeset}) do
    IO.inspect(changeset)
    conn
    |> put_status(:unprocessable_entity)
    |> put_view(MyapiWeb.ChangesetView)
    |> render("error.json", changeset: changeset)
  end

  def call(conn, {:error, :not_found}) do
    conn
    |> put_status(:not_found)
    |> put_view(MyapiWeb.ErrorView)
    |> render(:"404")
  end
end

My question is regarding the def call(conn, {:error, :not_found}) function in fallback_controller.ex. When it is called ?

• When a User is created or updated which is not valid, def call(conn, {:error, %Ecto.Changeset{} = changeset}) is called. I know how fallback with with works.

• When show or delete function in user_controller.ex is called with the wrong user_id, Ecto.NoResultsError is converted to 404 response.

So, what is the point of def call(conn, {:error, :not_found}) function in fallback_controller.ex ? When is it called ?