Why i need to install ar-core preview apk service before running hello-ar sample.if i dont install the ar-core preview apk the hello-ar sample app getting crashed.
You have to install the ar-core preview apk because it provides the tracking service/library. Shipping this separately (in the future, maybe via Playstore or as part of the OS) makes it easier for Google to update it - otherwise, each app developer would have to update their app themselves, e.g. to increase the device compatibility.