I never heard of these before. Marantz made a tuner or two that had scopes in them for optimum FM performance. You could see multipath and see if your tuning were dead on. It frequently drifted in the old days.
I found this:
http://marantzpassion.blogspot.com/2012/08/the-mcintosh-mi3-performance-indicator.html
I know a few tuners used to have o-scope outputs so you could do the same with an external scope. I once had a Heathkit oscope which was small for a tube unit, and low bandwidth, but plenty fine for this purpose. I used it for that and trouble shooting. Picked it up for $10 at a yard sale somewhere.
I guess this is just the same idea optimized for audio in a pretty case. Use for FM, use for testing, use for checking vectorscope like imaging of stereo signals.
http://sportsbil.com/mcintosh/Scope/MPI4/MPI4_own.pdf
Owners manual here. You kept it connected so you could see how your system was performing including phono, FM, and power amp. Combine with test record signals and you could do a system check at any time. I guess you could use a switchbox, and something like the $79 DAC and a computer for the same purposes today if you wished.