It used to reduce the signal strength

variable attenuator in microwave test bench is used to attenuate the amplitude of wave traveling through d rectangular waveguide

A variable attenuator is a device used to adjust the signal strength of a radio frequency or optical signal. It can be used to reduce the intensity of the signal by an adjustable amount to prevent overloading or distortion in the receiving equipment. Variable attenuators are commonly used in communication systems, test and measurement equipment, and audio systems to control signal levels.

Annenuator is a device which attenutes or reduces level of a physical parameter. e.g. an optical attenuator is used to reduce level of an optical signal in an optical fiber or an RF attenuator on a coaxial cable can be used to reduce RF signal level before giving it to a test equipment. - Neeraj Sharma An attenuator is an electronic device that reduces the power of a signal without appreciably distorting its waveform. An attenuator is effectively the opposite of an amplifier, though the two work by different methods.

Due to No of connectors, No of splicing, Quality of Fiber / connectors, Not using Attenuator according to the distance, by using dirty attenuators, etc.