Google Photos has quickly rocketed to the one billion user milestone, about four years after it was spun out of Google+.

It’s in good company: 8 other Google products—Search, Android, Chrome, Gmail, Google Drive, Google Maps, Google Play Store, and YouTube have all hit or exceeded that same milestone. And on Android, which now boasts 2.5 billion users, three apps—Maps, YouTube, and the Google app—have been downloaded over 5 billion times.

Fast Company has a nice overview of how Google Photos was built and why it is so successful. But the key here, I think, is that Google simply built a better version of something that customers were already doing. And users have flocked to the app as a result.

A big part of being better is offering free storage to all users. And while photos for non-Pixel owners can be “compressed,” most smartphones take lower-quality photos than Google’s compression bar anyway. For most, this is simply free storage.

And the service itself is excellent, too. Aside from offering automatic backup from smartphones, it provides regular AI-based treats to its users in the form of automatically-generated stylized photos, “Then & Now” shots that compare people in a recent photo to a photo of the same people together in the past, photo slideshow movies of recent events, and “rediscover this day” prompts that surface favorite events from the past.

Put simply, if there’s a service that deserves to have one billion users, it’s Google Photos.