Because new arrivals inject new money into the local economy. A new arrival buys products, pays for services etc. Their foreign dollars often also go further - meaning they can inject more vs a local, who - by nature of the problem - can't hence the need.

You could argue a tax break for locals would do the same thing, but it's not quite the same, as locals paying less tax may tend to choose saving any extra cash rather than change their spend-habits. Foreigners are bringing new money with existing habits.