Why Paging need RNTI scrambling?

Specs & Protocol Testing
1

Hi Experts.
Since paging comes on a separate channel, PCH, why does it need RNTI scrambling?
I thought this was the basis of not needed an RNTI for MIB which has a dedicated BCH channel.

2

UE have to read paging messages and decode TMSI for it or other UE.
paging and SI have fixed RNTI.

3

Is it TMSI or P-RNTI from UE will decode from paging?

4

P-RNTI is to decode PDCCH if has PDSCH scheduling for paging.
TMSI is L3 RRC messgae decoding.