How anyone could have thought this was a good policy, I cannot imagine. People migrate to your country from elsewhere and can't find a job that pays what they want or need, so the government says, hey, no problem, we'll supplement your income.
Remember this fundamental principle: When you subsidize something, you get more of it. So if you want lots of people coming into your country who can't earn enough to live without taxpayyer subsidies, then by all means, offer those subsidies! They'll come. And they're coming to Britain in such large numbers, Prime Minister David Cameron wants to put an "emergency brake" on it. So why not just put a stop to it altogether? Well that gets tricky, because Britain is part of the European Union, and one component of the treaty that established the EU says member states are supposed to extend their welfare benefits to people who migrate from other member states.