Getting a Sub Card is a great way to maximize your payments for your favorite restaurant. Every order allows you to earn points which can then be used to redeem prizes. You can also receive exclusive offers for members and other benefits. If you are already dining there a lot anyway, then you might as well get something out of your loyalty. The company sees you and aims to reward your patronage.
The card should be registered to obtain all the benefits. Look for the online page in their official site. This will probably come preloaded with a certain amount which you can use when you order. As this runs out, you can reload and reuse as much as you want. You can check your Subway Card balance by going online or by downloading the app on your mobile phone.