hah thats a very good question. I also don't know whats the purpose actually. to drain water maybe? or to recirculate the water when doing engine flush or servicing?
yes i agree with u. in fact the stopper that gave away on my previous ride was not on the radiator...it was on the engine, below the intake manifold, facing the firewall...quite hard to dismantle it.
Look out for those other tubes too. If the stopper rubber can harden and crack, those will also. The tube use to heat the inlet manifold auto choke, the U turn bypass tube and so on....