I own one pure attenuator, a transformer-based passive device with a volume pot (RCA L/R in & RCA L/R out). It’s sole design objective is to attenuate line-level signal. In my experience, attenuators are less about impedance than voltage: specifically, they reduce voltage. There are 2 scenarios where an attenuator can be helpful:
1 - In a system w/no preamp or DAC/pre having a volume pot, an attenuator’s volume pot can provide that necessary volume adjustment. For example, if you have no preamp but wish to connect a CD player or DAC directly to a trad stereo amplifier (powering speakers), you must insert a volume pot between these devices or the amp would receive full voltage output from the DAC & blow up the speakers.
2 - A more subtle application is where there’s already a volume pot in the system, but still, there’s too much system gain–forcing that volume pot into the low/non-linear range of its operation. I have this issue in one iteration of my desktop audio system. My DAC outputs 2.5 volts, 25% more than the usual/nominal 2.0 volts most single ended DACs output. If I connect this DAC to a headphone amp/preamp with standard volume control & no other gain adjustments, that volume control never gets above 9:00-10:00 AM on the dial, due to the high incoming voltage from the DAC, the result being too little effective range in the pre’s volume pot near its center, the most linear portion. The solution is to put the attenuator in-line between the DAC & the preamp, and use its volume pot to bring the DAC’s gain down by 25%, resulting in a more normal range of motion on the pre’s volume control.