I understand the concept of "Split Horizon" that an iBGP router A learns an external route from an iBGP router B, thereafter, iBGP router B will not advertise the same route to other iBGP routers.
However, why would an iBGP router NOT modify the NEXT_HOP address when advertising an external route to other iBGP peers? I understand it is a rule of iBGP but what purpose does it serve?