Google Play Services become the first App on Google Play Store that has been downloaded 5 billion times. This achievement does not mean that there are 5 billion active users of Google Play Services; it just means that it was downloaded 5 billion times which also includes the devices that are not active anymore.
Excluding a few devices in China, Google Play services are a pre-installed App on all Android devices. So when you open your phone, you connect it to the internet. When you are asked to add your Google account details. As you share your details, Play Store automatically downloads Google Play Services in the background. This also tells us that since Android came into existence there have been at least 5 billion active Android devices.
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It is important to install Play Services as many applications depend on them to perform properly. It is an essential App for Android that ensures that Android and its Apps work correctly. If you uninstall the Play Services many apps wouldn’t function properly. They will show you an error and you will be asked to install and update Google Play Services to run the apps properly. The play services allow users to add new, exciting features to the older devices. So for many features users don’t need to completely update the OS, they can only update Google Play Services and keep getting up-to-date features.