2 I would not, however, regard this reason as the most important one: completing the single market was arguably the most important motivation, and promoting the international role of the euro is nowhere to be seen in the Treaty.
Second, there are information synergies between the oversight of payment systems and the supervision of banks. The ECB is not only responsible for the oversight of all large-value payment systems operating in the euro area, but also the owner of the TARGET2 system, the largest of such systems. Third, central banks have a lot of expertise on the financial sector.