SGW node in LTE buffers the data since down link tunnels towards eNodeB would not be available or in active in case UE is in RRC IDLE mode. In RRC idle mode, UE does not maintain radio connection with eNodeB but it continues with NAS level context that's why network also maintain context at MME, SGW and PGW as it is.
eNodeB PDCP layer for an UE comes into picture only when UE is in RRC Connected mode.
One more thing I would like to add here is when SGW does buffering for down link data at the same time it notifies MME. As result, MME triggers paging for UE.
