The opening and closing of the injectors create pulses in the fuel rail, which can lead to unstable fuel pressure. This damper should be used (especially with high flow injectors) in order to achieve a safe and consistent rail pressure. The internal diaphragm will absorb oscillations and smooth fuel pressure.
This 6AN male adapter is used to integrate a high flow check valve on the outlet of many fuel pumps. The valve outflows the integrated check valve that comes standard on many fuel pump outlets. Furthermore, decreasing this restriction allows the pump to operate at less current.