(Spoilers Main) Why would the Iron Bank even do this?

Them giving money to Stannis might not make sense. Hey bro give me lots of gold to hire mercanaries and ill go north and make them endure the harshest winter ever so we can defeat these wildings outside the wall and then ill march all the way south taking over each kingdom as i go and then i'll pay your debts back

why would they even care about wildlings crossing the wall? if anything it's a plus because it means the 7 kingdoms that arent paying you back are under threat