How does solo piano help you evaluate audio gear?

IME, very important when evaluating a turntable.  

Not to be facetious - I think solo piano is an important instrument to evaluate equipment with if you like to listen to a lot of solo piano.  I would venture a guess that the majority of audiophiles cannot, either by budget or space constraints, own no-compromise speakers that excel on all genres of music.  Assuming such a thing exists.  So most of us make some compromises and IMO, that's best done by choosing speakers that make the music you value sound good to your ears.  I'm not saying "accurate" or "true to the recording" because, unless we produced the recording, we can never know what that is.  So I'm just saying it has to sound good to us. So for me, piano is important.  BTW - If you want some nice piano music, try Bach on a Steinway and Dr. John Plays Mac Rebennack.  Both excellent solo piano recordings, IMO> 

@testpilot @chayro Thanks. Piano is important to me, too, so I'd listen for that reason.

I do sometimes listen to music which is not the genre of what I usually listen to, but which nevertheless demonstrates something valuable about the system's abilities which would still be a benefit to me for the listen I do prefer. E.g. Argentinian folk music is not necessarily my thing, but I love American and English folk music. So, clear crossover value of hearing a good Argentinian folk recording as a test.

That's a long-winded way of asking (as I did meant to in my OP) whether the "piano test" has a value which goes beyond the subset of music lovers who like piano.

Piano is probably the single best instrument to use as a reference in that it can deliver good dynamics across a wide range of frequencies, but unless one was highly biased towards listening exclusively to piano music (what??) I would not rely exclusively on it. But include it for tests, yes. Piano recordings vary widely in nature of the sound so find good ones to use if desired.

Well-recorded piano can show a lot about a system. Assuming it was well-recorded. It covers virtually the entire musical pitch spectrum and has a huge amount of weight when captured accurately, not to mention the inner detail of the hammers striking the strings, the sounds of the pedals and the bench creaking, which it often does. So, assuming you know what to listen for, piano can be an excellent tool in selecting a speaker. But it is one of many tools. If I were forced to pick only one, I personally would choose the human voice.