Hi Divyansh,
In my understanding, InvIT is just a monetization route where Powergrid can get back their invested equity along with their returns and then decide whether to give it back to shareholders, pay down debt or bid for more projects. Its important to note that Powergrid only monetizes TBCB assets which are acquired via competitive bidding (where ROEs are lower) compared to regulated assets where they make 15.5% ROE + incentive income.
About the bidding details, please refer to the Motilal report on this thread. I dont have enough expertise to give an informed answer.
The main reason for me buying Powergrid was that they had large CWIP which was supposed to come onstream and lead to improved freecashflow which should be returned as dividends. This thesis actually played out and I exited last year. You can see my thoughts below.
The reason for my exit was mainly around growth concerns as most of their large assets already got commissioned. Hope this is of some help 
Disclosure: Not invested (no transactions in last-1 year)