TPS56637: Design questions by customer

Hello, 

I have a customer asking the following:

In my new design I use DC/DC (12V to 1V/6A) TPS56637RPAR.

According to Datasheet and Evaluation Board Guideline  The AGND should be designed separated from PGND and connected at one point.

Are this must or as a option?

What purpose of this recommendation?

Is this recommendation could be to my detriment when used 8 layers PCB and many high speed ICs?

Thermal problems?

 Report_12V_to_1V_5A.pdf

Thanks in advance