There are several reasons why companies want to generate a new userId for each sessions. How this works on web is outlined here. Please note, when you decide to randomize the userId as outlined below, the metric Unique Users will be equal to the Playsmetrics.

On Android

On Android there is setting in the BitmovinAnalyticsConfig called randomizeUserId. If this setting is set to true then every time the collector is attached a new userId will be generated. If a user is watching playlist, the user will have same id for every video in playlist.

If this setting is set to false then Settings.Secure.ANDROID_ID will be used as userId, which is not a random value!

On iOS

On iOS the behavior is consistent with the one on Android. In the BitmovinAnalyticsConfig there's a setting called randomizeUserId. If this setting is set to true then every time the collector is attached a new userId will be generated.

If this setting is set to false then a random ID is generated, stored in the UserDefault Store and retrieved every time the user starts a session.