适配器设计模式的解释

以下是我对适配器设计模式的理解

你有一个遗留的支付系统:

class LegacyPaymentSystem {
    public function pay($amount) {
    }
    public function refund() {
    }
}

你实现了一个新的支付系统:

class PaymentSystem {
   public function __construct() { 
   }
   public function payAmount($amount, $currency) {
   }
   public function refund($payment_id) {
   }
}

您使用适配器来桥接两者。有时你想使用旧的支付系统。

 class PaymentSystemAdapter extends PaymentSystem {
    public function __construct($legacyPaymentSystem) {
       $this->legacyPaymentSystem = $legacyPaymentSystem;
    }
    public function payAmount($amount, $currency) {
         $this->legacyPaymentSystem->pay($amount);
    }   
}

现在客户端可以做:

class Client { 
    public function process($amount, $currency) {
        $legacyPaymentSystem  = new LegacyPaymentSystem();
        $adapter = new PaymentSystemAdapter($legacyPaymentSystem);
        $this->pay($adapter, $amount, $currency);
    }
    public function pay(PaymentSystem $paymentSystem, $amount, $currency) {
        $payementSystem->payAmount($amount, $currency);
    }
}

我的问题是为什么?为什么我们不能直接调用遗留支付系统呢?