Google Play Services started as a way to solve Android's fragmentation problem: instead of adding APIs to new Android versions, they were added to Play Services, which was bundled with almost any Android device and updated automatically by Google. Play Services became a clever tool that allowed Google to add new features to Android without waiting until phone manufacturers update their software.
Now it looks like Google can deliver timely security updates using the same software bundle. Apparently, Google uses a "Dynamic Security Provider, which offers an alternative to the platform's secure networking APIs that can be updated more frequently, for faster delivery of security patches."
Google Play Services is updated every 6 weeks and about 93% of the Android devices have the latest version. "By updating this application, Google can keep as many Android devices and Android users up to date with software, services, and security issues," mentions Ewan Spence from Forbes, who suggests that Play Services is also a way for Google to control the Android platform and make it more difficult to fork Android, since Play Services is not open source. "By being in complete control of the access to Google Play Services, and by association the other closed source apps that consumers feel are integral to the Android experience, Google acts as a gatekeeper to the successful Android world."