What is merit of using portal logon group ??

hi experts,

EP and java system has logon group function according to following url.

http://help.sap.com/saphelp_nw04s/helpdata/en/a9/775a421b5ec153e10000000a1550b0/content.htm

So I setup portal logon group containing central instance and dialog instance.

But the url for first access is as same as in case not using logon group.

but in background other app server seems to be used for user dialog process.

So I was expecting this function was same as ABAP logon group.

My setting could be wrong but Which situation or server config can get

merit using portal logon group function??

Regards

MASAKI