Insuring that when the user resets his Advertising ID, the system isn't ...

...using his previous one for further requests and instead creates a new user.

This is needed because of the Google Policy regarding advertisement:

Respecting users' selections. Upon reset, a new advertising identifier must not be connected to a previous advertising identifier or data derived from a previous advertising identifier without the explicit consent of the user. Furthermore, you must abide by a user’s “opt out of interest-based advertising” setting. If a user has enabled this setting, you may not use the advertising identifier for creating user profiles for advertising purposes or for targeting users with interest-based advertising. Allowed activities include contextual advertising, frequency capping, conversion tracking, reporting and security and fraud detection.

https://play.google.com/about/developer-content-policy.html