Private bankers lend money & collect interest. They get more interest from weaker borrowers. When a borrower can't pay on time, the public instead must pay those debts.
Sound crazy?
Not for a lender bank. Look carefully at this "can't lose" excellent business plan - - politicians dispense taxpayer funds to private banks (or to weak borrower nations) and make you pay-in involuntarily.
The stronger of the world are already well-served with good banking relationships. So what can a hungry bank do? Push the marginal borrowers into high-priced debt. Any problem and Joe Public will bailout the bank for the note.
This happened in the USA with the housing bubble, and earlier with savings & loan companies; now the public shoulders sovereign debt of Greece. Banks have played the politicians and ultimately the taxpayers to backup bankroll their lending.
Banks have not "accidentally" found themselves in this situation. As financial brokers it is fundamental to their business plan. Buying-selling selling-buying, they collect whatever happens.