Tim Cook held an iPhone 6s Plus high in the air.

The CEO of Apple had a grin a mile wide, and he has a good reason to be excited. That actual phone--it's the Rose Gold version in the photo above--is the one billionth sold, a milestone few tech companies can boast.

"The iPhone has become one of the most important, world-changing and successful products in history. It's become more than a constant companion. iPhone is truly an essential part of our daily life and enables much of what we do throughout the day," he said to a cheering crowd.

I agree with every word he said.

The reason the iPhone has racked up so many sales over the years is due to one simple reason: The phone works. It doesn't crash, the apps load quickly, you can find what you need, you get your stuff done. Even when new apps debut, like one I've tested called Lola that helps you find hotels and flights, they work without glitches.

About one billion people agree with me. I decided to ask some of them to explain why they think the phone is so useful for their daily work and why it is so popular.

Jessica Gottlieb, an automotive journalist at TheOtherPTA.com, told me she can't live without the iMessage app, which chimes all day as she works--on her computer. This feature, which Apple calls Continuity, is indispensable because it fits right into her workflow. She says her kids can also see when she has received a message.

I've used Continuity myself many times and it's awesome to text from a Mac, although that's the catch--it requires a Mac. (Apple, please make this work on a PC and Chromebook.) There are somewhat similar offerings from Samsung and Motorola (now owned by Lenovo) where the phone can sync with your laptop, but they are not as intuitive and easy and they also require a fairly locked-down ecosystem.

Edward Yang, a Managing Partner at Firecracker PR, told me his main reason for using the iPhone was due to how the Android models he's tested were always having problems. He would patch and patch, but a new issue would crop up. I'm the same way. There are features on many HTC and Samsung phones I can't live without, but Android tends to be a little more "open" and therefore a little more temperamental. One good example of this is the Fluenty app. It only runs on Android, which is a differentiator, and it taps into your notifications and messaging. (The app lets you send stock answers to messages; an A.I. determines which answers work best.)

It's a cool app and one of my favorites, and it doesn't work on the iPhone. Yet, it also means the iPhone won't crash when developers try to intercept your text messages.

In fact, when I've dissed Android in the past and mentioned the stability and user-friendly features of the iPhone, the Android fans come out the woodwork. "I can run my own battery management app," they say. "I can load a custom app for my business without having to go through the Apple vetting process." "I can customize the interface." "I don't have to put up with the locked down iTunes model."

I get that. I'm not 100% on the iPhone bus and never have been. My main reason for choosing it as my main work phone is fairly simple. I need the reliability. When I travel, I often regret bringing a laptop or phone that doesn't quite meet my needs, crashes, or has other problems. "Oh shoot, this phone doesn't last nearly as long as they said it would in the marketing literature." It's painful. And, it makes me less productive.

There's even a company that has even built their software around Apple products.

"Apple's sale of its one billionth iPhone is a major milestone, no doubt," says Dean Hager, the CEO of JAMF Software, an iPhone and Mac management service based in Minneapolis. "You could argue the iPhone's success is the result of Apple's innovation, apps, and alliances. Importantly, Apple developed its iPhone by strategizing around the user--by humanizing technology. The result is a product people love to use, and a redefinition of what we expect from mobile technology."

That's quite a vote of confidence and one that's echoed across many companies across the globe and many happy users. Many, many happy users. The biggest reason I'm a fan of what Apple has accomplished is because it makes us more productive and maybe even a bit happier in our jobs. Kudos to them.