What is the use of adding MAC-I to PDCP PDU format for SRB?

Hi All,
I have one query for pdcp layer functionality in LTE.
As mentioned it’s used for integrity of c -plane data.
This integrity function done by security mode command as mentioned
Query: What is the use of adding MAC-I to PDCP PDU format for SRB?

As already integrity function done by security mode command.
I think MAC-I octet indicates only integrity related key.