Now I got the idea!!!
To transfer RRC messages (messages generated at RRC layer itself and NAS messages) between UE and UTRAN.
RRC messages are transferred by the lower layers through Signaling Radio Bearers (SRBs).
Thus, establishment of RRC connections mean establishments of different SRBs.
Only a single connection.
Establishment is needed in order to transfer NAS signal messages. That is, messages are exchanged between UE and CN.
However, note that this NAS signal messages still use SRBs between UE and UTRAN!!
The upper layer signalling messages (exchanged by the UE and the CN) may request the initiation of a user service (beloning to certain service class and with different requirements) .Then, RB will be established. RB is a user data.