What is use of MAC CE in DL?

s-TMSI
value UL-CCCH-Message ::= 
  message c1 : rrcConnectionRequest : 
        criticalExtensions rrcConnectionRequest-r8 : 
            ue-Identity s-TMSI : 
                mmec '00101000'B,
                m-TMSI '11110100 00000000 11000111 01110100'B
               ,
            establishmentCause mt-Access,
            spare '0'B

@ankgitm, @Prasoon_P, @marcengo, @Shalab_Tripaathi:
I was talking about this UE identity UE send in msg3 in MAC header CE