Why do I need an Account Aggregator account on PhonePe?

With an Account Aggregator account,

Note: An Account Aggregator ID (AA ID) will be linked to FIP accounts to help identify and fetch relevant financial information.

Related question(s):
How do I register and link my bank account on Account Aggregator?
Will I be charged for using Account Aggregator services on PhonePe?
​​​​​​​What is AA ID?