Case Statements for Error Handling in Ecto Changesets

Hi all! Going through the Ecto docs and trying to understand something-

Makes sense to me for their suggestion to use a case statement like

case Friends.Repo.insert(changeset) do
  {:ok, person} ->
    # do something with person
  {:error, changeset} ->
    # do something with changeset
end

for the error handling- just not quite sure how that’s implemented with the an existing Schema changeset like-

def changeset(person, params \\ %{}) do
  person
  |> Ecto.Changeset.cast(params, [:first_name, :last_name, :age])
  |> Ecto.Changeset.validate_required([:first_name, :last_name])
end

How exactly is this case statement wrapped in def changeset?

Additionally, in this case statement: what am I doing with the first {:ok, person} tuple? I understand with the {:error, changeset} tuple I can return something like-

traverse_errors(changeset, fn {msg, opts} ->
  Enum.reduce(opts, msg, fn {key, value}, acc ->
    String.replace(acc, "%{#{key}}", to_string(value))
  end)
end)

Thanks for any help