Apple's highest-profile launch this year hasn't been a new phone or tablet — it's been a credit card. Investors and analysts have placed a lot of hope in the card as a high-profile new product in Apple's online services business that management has highlighted as a growth engine for the company.

But the Apple Card could end up being a smashing success even if it doesn't contribute a meaningful amount to the company's services revenue or end up being Apple's beachhead in the financial industry.

Rather, the Apple Card is all about keeping users glued to Apple's most important product: the iPhone.

The Apple Card can only be signed up for on an iPhone. Sure, there's a metal card, but the primary interface for the credit card is on your iPhone — and that includes for paying bills.

If you lose your iPhone, you have to pay your bill from another iOS device, or you can call an Apple support phone number, which will connect you to Goldman Sachs to pay your bill, according to Buzzfeed. There's no web portal on Apple's website.

All this means is that if you have an Apple Card, it's going to be hard to switch to Android, at least before you pay off your balance. The credit card makes the phone much stickier.

And people with iPhones are much more likely to buy AirPods, Apple Watches and apps for their devices, all of which essentially require an iPhone to work properly, and all of which feed back into the cycle — when the person with all those Apple products wants a new phone, they'll buy an iPhone, because it works with all of their Apple products.