Being in the good graces with the bank is a big, BIG deal. Means their credit is good again. Means they can buy extra provisions and hire sell swords. Make bribes and hire assassins. Gold wins wars. In order for Dany to bring money from her kingdoms in Essos, she has to bring it by ship. Euron could maraud the narrow sea and pick them off along the way.
Also, the idea of Dany just flying to the wall seems pretty easy except the dragons are not Jets. They have to flap their wings the entire way. It's a thousand miles from Winterfell to Kingslanding. Farther to Dragonstone and farther still the wall to Dragonstone. Sure, it's faster than any other form of transportation but Dany would be out of communication for days if she flies there and back.