Need of sending UE Capability Enquiry:
The eNB broadcast Cell specific information in the broadcast messages like MIB and SIB. This information is common to all UEs.
Now once the UE is Authenticated (With Authentication Request and Response messages) and Security Mode Command is successful, the eNB tries to Reconfigure UE as per UE capabilities and for this eNB sends UE capability information Request to UE to which UE responds back with UE Capability Information Response message.
Based on UE capabilities eNB then Reconfigures UE using RRC Connection reconfiguration procedure.