Why match request parameter using strings?

Questions & Help Questions
1

Although in programming phoenix, the author tries to explain why not match on %{:id => id}. I still do not understand. Can someone explain this to me? I thought it will be better to match on atom since the key in the connection struct is :id

def show(conn, %{"id" => id}) do
    user = Repo.get(Rumbl.User, id)
    render conn, "show.html", user: user
end