Earlier quoted context omitted.
Either for humanitarian reasons, or because Germany has an ageing population and desperately needs young workers to support its welfare state.
It is one. The majority of migrants, qualified or not, have a hard time getting work permits. Finding work, low paid but paid, isn't the main challenge. Doesn't do any good if you aren't allowed to take that work so. Also, our social systems are doing well. Thanks for worrying.
If you include pension systems, not so much. "Altersarmut" ("poverty of the old age") was not a widely used word in the 1990s, now it is a frequent topic in German media.
In many other European countries, old people live in their own properties with mortgages long paid off, so they aren't directly touched by rent increases. But Germans, for some reason, mostly like to live in rented flats and have a comparatively low home ownership rate, even in their old age. And the rents have grown quite a lot in big German cities. This means quite a squeeze for the elderly.