A device ID is a unique string of letters and users to identify individual mobile devices. Every smartphone and tablet device has a unique Device ID and that information is collected when a user downloads an app in order to get pre-install and in-app behavior data including clicks and purchases. This is why Device ID is significant to understand user behavior and to make data-driven decisions accordingly. 

Device ID is also important for gathering attribution data and this data is crucial to detect some types of mobile ad fraud including ID mismatches. 

Android – Google Advertising ID (GAID)

iOS – Identity for Advertisers (IDFA)