Significance of PSG in WSR

Hello Gurus,

What is the significance of creating PSG in SAP?

Like if we have two PAs Maharashtra and Karnataka and subareas like Mumbai, Pune of Maharashtra and Bangalore of Karnataka.

Then can we directly assign PSG to Company Code which contains all these PAs and PSAs?

If yes then why are we using PSG(which is simmilar to CtryGrp) in maintaining WSR instead of CoCode?

Also why CtryGrpg and PSG are same, like for grouping Indian PAs, PSG would be 40? Can we assign different ID to PSG?

Why is it not considered by SAP?

Regards,

Yogiraj