Since Modify bearer request/response message can be used for multiple purposes (mentioned in the 3GPP TS 29.274) but As per your question It is being used to communicate downlink tunnel(s) information to SGW during the attach procedure. During attach procedure when MME executes create session procedure involving SGW and PGW, it comes to know about the UL tunnel information, which is sent to eNodeB using S1-AP "Initial Context Setup Request" message.
Once SGW receives Modify bearer Request message from MME, it can forward downlink data to eNodeB.
Hope I answered your query.
