Prevents inbreeding of ideas. Some of those immigrants may be a heretic monkey. This passage from the anime "Irresponsible Captain Tylor" [1] explains it better.
"Do you know about monkey tribes? Monkey tribes are very organized, with a leader at the top. Now, once every few years without fail, a renegade monkey leaves his tribe and tries to join another one. The members of the new tribe greet the renegade monkey by beating it to a bloody pulp. And yet those renegade monkeys, those heretic monkeys if you will, are a providence from nature. Why? Because they keep the bloodlines from becoming too thick, from too much inbreeding within the tribe. So, you could say that those heretics are essential for survival. Heretics are essential for survival."
"Do you know about monkey tribes? Monkey tribes are very organized, with a leader at the top. Now, once every few years without fail, a renegade monkey leaves his tribe and tries to join another one. The members of the new tribe greet the renegade monkey by beating it to a bloody pulp. And yet those renegade monkeys, those heretic monkeys if you will, are a providence from nature. Why? Because they keep the bloodlines from becoming too thick, from too much inbreeding within the tribe. So, you could say that those heretics are essential for survival. Heretics are essential for survival."
[1] http://greybeta.dreamwidth.org/179747.html