I find some information on internet of this question "What pros and cons of combined S-GW/P-GW?".
However it did not write the reasons.
I find this answer in http://www.cisco.com/c/dam/en/us/solutions/service-provider/mobile-internet/whitepaper_c11-577763_v1.pdf
It said that combined S-GW/P-GW have these advantages:
Reduced signaling overhead (S5 and S8 would be internal)
Fewer hops on the bearer path
Reduced signaling on the S7 interface
Lower session requirement for the PGW
I cannot understand why it can reduce signaling on the S7 interface.
and although I probably know other advantages' reason, I am not sure.
Someone please tell me.