Apple CEO Tim Cook speaks during The Wall Street Journal Digital Live (WSJDLive) conference at the Montage Laguna Beach, California, October 19, 2015. REUTERS/Mike Blake This has been a huge year for Apple, the most valuable company in the world, with the launch of three major products: The Apple Watch, Apple Music, and the massive iPad Pro tablet.

Despite the hype, all three of those products earned mixed reactions.

Compound that with the repeated and persistent reports that Apple is going to make an electric car, and a lot of pundits are wondering just what the heck Apple thinks it's doing.

Well, it's pretty simple.

If you ever want to know why Apple does what it does, look at it through this lens: "How does this benefit the iPhone?"

Removing the brakes from the train

Today, Apple iOS may not be the most popular smartphone operating system in the world — Google's Android holds that distinction — but the product based on it (the iPhone) is the most profitable tech product the world has ever seen.

In fact, if you broke out just Apple's $31.4 billion iPhone revenue from the last quarter, it's higher than any other tech company's total revenue over the same period.

If iPhone shipments slip, even a little, it's going to cause real pain to Apple's sales and profits. So it's all-important that the iPhone keep going.

But a big reason why iPhone sales aren't slowing down is because Apple is removing any reason to hop off the train.

A MacBook Air with iPhone. Flickr/Alejandro Pinto Apple's goal is to create the best platform experience in the world, so each new Apple product you buy improves the experience on all the other Apple products you already own — particularly the iPhone.

For instance, both the iPad and the MacBook get even better if you also have an iPhone, since you get the ability to sync texts and send photos across devices.

Conversely, it means that if you get an Android tablet or a Windows PC, you lose those iPhone-related superpowers. This is extraordinarily clever. It means that users have a series of good reasons to own all Apple devices. When it's upgrade time for any of these devices, it's a serious reason in favor of buying another Apple device, whether it be phone, PC, or tablet.

Now, extend that logic out.

The platform effect

The Apple Watch, which seems to be a sales disappointment, is still very likely the best smartwatch on the market. And if you want to use one, you need an iPhone. Not everyone will want one, but for those who really want a smartwatch, it's one fewer reason to leave the iPhone world and one additional reason to join it.

The new Apple TV, which is a refresh to a product that the company had all but forgotten about, now also has the ability to run a selection of iOS apps.

Better yet, if you already own an app for your iPhone, the attendant Apple TV version is also free. It means that if you have an iPhone, the Apple TV looks better. If you have an Apple TV, the iPhone looks better.

The same goes for the iPad. Maybe nobody is replacing their old iPads as fast as Apple shareholders might have hoped, but it's still selling in the tens of millions a year. And if you have an iPhone, you probably already also have a bunch of iPad apps.

Similarly, the iPad Pro exists because Microsoft has proven the market exists with its Surface Pro 4 tablet/laptop hybrids. Apple wants as few reasons for people to go outside the Apple iOS world as possible. Getting work done on an iPad Pro with Apple Pencil stylus. Melia Robinson/Tech Insider Looking forward, the Apple Car is a little more esoteric. But if you look at it as Apple looking to protect the future of iOS in the automotive market, it makes a little more sense. If it succeeds, Apple would own the operating system in a car that it also owns, extending the dominance of iOS to the roads, and making an iPhone an even smarter purchase. The same goes for HomeKit, Apple's platform that connects the iOS ecosystem to various devices in your home.

Apple Music is kind of an odd duck, since it has an Android version and works on the Windows version of iTunes. But it's integrated deeply with the iPhone and iPad versions of the built-in Music app, which means that Apple devices are still the best place to run the service.

All of which is to say: It doesn't matter if any one Apple product succeeds or fails, so long as the iPhone continues to succeed. And with each passing day, Apple makes more and more defensive moves to keep anyone from ever considering anything else.

Just remember that for this strategy to work, Apple has to keep coming out with the best versions of whatever it comes up with. They don't have to be massive hits. But if they're flat-out bad, customers will start looking outside of Apple's little world. And maybe they'll start looking for something other than an iPhone to go with it.