Apple has an iPhone with an edge-to-edge display and an iPad that can replace your laptop. These devices established their place in the cultural lexicon long ago, offering convenience, power, and groundbreaking ecosystems that drive entire industries. But neither is as important as the Apple Watch.

Apple’s wearable has slowly grown into the most popular wearable in the world, despite the fact that it lives behind a walled garden. In other words: no iPhone, no Apple Watch. But its seamlessness with Apple’s ecosystem isn’t what makes it so great—though its ease of use doesn’t hurt.

What began as a way for iPhone owners to screen notifications has evolved into a powerful activity tracker. And that’s what makes the wearable such an important part of Apple’s future. Forget the fact that it can make and receive phone calls, or stream music without connecting to an iPhone.

The Apple Watch excels because it encourages users to be more active, whether they’re taking a walk or swimming laps. It quantifies your slightest movements—it’ll even remind you to stand up after a period of inactivity, or quickly meditate for relaxation. As Apple says, it’s the “ultimate sports watch.”

And, new to watchOS 4, Apple’s wearable can connect to gym equipment, making it easier to track metrics such as heart rate, speed, and calories burned. It’s a helpful tool for casual exercising and people who are training for a Triathlon, thanks to its ability to track indoor and outdoor workouts.

There’s even a Smart Coaching feature that acts as a personal trainer. The extra motivation helps, especially on those days when all you want to do is go home and veg on the couch. There’s something so thrilling about seeing the Apple Watch’s activity rings close after hitting a goal.

But even more than tracking activity, the Apple Watch’s real potential lies in its approach to health.

What makes it such a powerful health tool is its ability to monitor your heart rate, call for help in an emergency, and tap into iOS’ massive developer community, which includes apps like Lose It! and One Drop, the latter of which allows users to track and share glucose levels to better manage their diabetes.

Apple is also planning to connect the Apple Watch to a heart study, which the company says will use data from the wearable to help identify irregular heart rhythms, “including those from potentially serious heart conditions such as atrial fibrillation.”

Not only can the wearable potentially help you become a healthier version of yourself, but it could literally save your life. And that’s why it’s so important. Not because it can send messages or play music or make phone calls. It could be huge for medical use cases.

The fact that Apple is aligning the device to become a tool for combating ailments and disease makes it something worth rooting for. By no means is it the perfect gadget, and the Apple Watch isn’t the only wearable that’s exploring health scenarios. But as the market’s most popular activity tracker, it has the best chance to change how people live in a meaningful way.

There are still hurdles the Apple Watch needs to overcome, like mediocre battery life and performance. But we like that the device is headed toward becoming a health tool rather than needless phone accessory. If you buy any device this holiday season, the Apple Watch ($329) is worth a look.