Yes i also came under same question when i was going through Handover Request message .
This optional id importance is there when ENB (HeNB) and MME connected through Gateway instead of Directly .
Here GW creates two context one towards ENB and another towards MME.
See If Source ENB is connected to MME through Gateway (see figure) .
So during Handover the Target ENB should know two ids.
1>The MME UE S1-AP Gateway id (so that he can use Target ENB and Gateway connection ) as
2> The MME UE S1-AP id which is the generally we send.
So the 2nd ID is needed and it is optional as it is used in this case only.
For more description u can refer http://www.3gpp.org/ftp/tsg_ran/WG3_Iu/TSGR3_70bis/Docs/R3-110241.zip