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

On Android

On Android there is a setting in the AnalyticsConfig 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 with a playlist, the user will have the same id for every video in the playlist.

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

On iOS

On iOS the behaviour is consistent with the one on Android. In the AnalyticsConfig there is 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. false is the default value.