Don't understand WooCommerce Paypal Standard Gateway - php

Question

So I'm building a custom payment gateway for WooCommerce which has been... challenging, mostly because I'm not a PHP developer so I have been learning as I go. I've started by dissecting the PayPal Standard Gateway which ships with WooCommerce. That gateway has an included class for handling IPN callbacks and I'm having trouble understanding why their code is set up the following way:
/**
* Constructor.
*
* #param bool $sandbox Use sandbox or not.
* #param string $receiver_email Email to receive IPN from.
*/
public function __construct( $sandbox = false, $receiver_email = '' ) {
add_action( 'woocommerce_api_wc_gateway_paypal', array( $this, 'check_response' ) );
add_action( 'valid-paypal-standard-ipn-request', array( $this, 'valid_response' ) );
$this->receiver_email = $receiver_email;
$this->sandbox = $sandbox;
}
/**
* Check for PayPal IPN Response.
*/
public function check_response() {
if ( ! empty( $_POST ) && $this->validate_ipn() ) { // WPCS: CSRF ok.
$posted = wp_unslash( $_POST ); // WPCS: CSRF ok, input var ok.
// phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores
do_action( 'valid-paypal-standard-ipn-request', $posted );
exit;
}
wp_die( 'PayPal IPN Request Failure', 'PayPal IPN', array( 'response' => 500 ) );
}
/**
* There was a valid response.
*
* #param array $posted Post data after wp_unslash.
*/
public function valid_response( $posted ) {
$order = ! empty( $posted['custom'] ) ? $this->get_paypal_order( $posted['custom'] ) : false;
if ( $order ) {
// Lowercase returned variables.
$posted['payment_status'] = strtolower( $posted['payment_status'] );
WC_Gateway_Paypal::log( 'Found order #' . $order->get_id() );
WC_Gateway_Paypal::log( 'Payment status: ' . $posted['payment_status'] );
if ( method_exists( $this, 'payment_status_' . $posted['payment_status'] ) ) {
call_user_func( array( $this, 'payment_status_' . $posted['payment_status'] ), $order, $posted );
}
}
}
The first add_action registers a hook with a WooCommerce action and I totally get that - it's a way to extend functionality in an external package. However, the second add_action( 'valid-paypal-standard-ipn-request'...) seems to simply call valid_response() - which is a local function - inside the next function check_response(). Why couldn't they just call valid_response() directly within check_response()?