Implementing reliable interfaces to the GCM and APN protocols is difficult. There are open source projects to do it, but even the best ones break when message quantity begins to exceed 500,000 at a time. These protocols also change frequently, for instance, last week Apple released a new protocol and deprecated their old one.
OneSignal provides marketing tools including A/B testing, segment targeting, localization, drip marketing, and conversion tracking.
OneSignal provides a single UI and API to deliver messages across iOS, Android, Amazon Fire, Windows Phone, Chrome Apps, Safari, Chrome Web, and Firefox.