I have a Schiit Kara preamp. It has three gain settings including a passive mode where the signal goes only through the RL volume control, and if the volume is wide open, not even that.
There are both SE and balanced inputs and outputs. I can connect multiple preamps, DACs, CD transport, etc. and output to multiple amps or powered speakers.
Selecting the correct input, gain mode, and volume setting I can quickly switch which combination of gear I'm listening to. It's very useful for quick comparisons though I do agree that extended listening is critical to a full evaluation.