The Treasury is the upgrade of the bank. It focuses on generating Money faster passively.
It is actually more efficient to upgrade the Treasury than it is to construct a second bank in any situation where money is needed. The amount of money generated does add up over time and works great with the Special Operations structure for missiles.
The Forge line of units such as the Phantom and A28-Gladiator are expensive in money when mass producing, the treasury can greatly aid in providing the economy.