It is typically used to connect a fluid line to a rigid, internally threaded hydraulic component.
A flexible hose or a rigid pipe may be connected to the union port.
[1] The main advantage of the fitting is in high pressure applications (i.e. more than 50 bar).
It also allows the pipe exit direction to be adjusted relative to the fitting, then the bolt tightened independently.
Banjo fittings are commonly found in automotive fuel,[2] motor oil and hydraulic systems (e.g.: brakes and clutch).