something about the power supply of STM32H745IIT6(LQFP176)

[RZHU.1]

I am designng my own board with STM32H745 and I am confused about the SMPS so I do my work according to my STM32H757I-EVAL board.

I have finished the design of the ciucuit about power supply but the VSS pins. I found the several VSS pins of my evaluation board are connected to "SMPS GND", why? Does it necessary?