krunal,
the benefit is that it is *simple* !
in effect, it means that there is one Routing Area per LAC, because a RA is defined by the following ID:
MNC+MCC+LAC+RAC
So even though the RAC is the same everywhere, each Routing Area is individually identified by its LAC.
So you just have to ensure that PS Paging is not congesting your PCH when having RAC = LAC, otherwise you would need to have 2 RAC per LAC.
Cheers
Pix