The Bank is an auxiliary building for Power Sales. It does not sell anything on it`s own, but improves the sales in adjacent tiles. It goes without saying that all viable adjacent tiles of a Bank should contain Offices.
The description in-game says that Bank improves sales by 2.5 times, but this is a misleading statement. What bank really does is add 1.5 times sale power of a building to it, flatly. So an office that has one bank next to it will sell 2.5 times the power. An office with two banks, however, will sell even 4 times, not 6.25 as one might expect.
To sum it up...
|Number of banks||1||2||3||4|