Bypassing Caps - what is it really about?

So I’m not talking about bypass circuits. I’m taking about what hi-fi people call "bypassing a capacitor", which is running two capacitors in parallel, typically using a small value to piggy back the main value.

Thanks for the answers! I would not have intuitively thought about this because the caps I’ve used (Audio Note, VCap, Jupiter, etc) all seem to have fantastic high frequency sound, even if their ESR measures this way or that. (I kind of wonder what kind of person uses VCap teflon or Deuland silver and still doesn't hear enough high frequencies detail) 

I do have a 350uF industrial filter cap on the preamp plate, and that seems like a target for bypassing to experiment with.

Also, from reading articles from various "tone" guys. they seem to bypass based more on blending the sound of their caps. For example I’ve blended electrolytics on each wave of the rectifier, it seems strange each get a different brand of cap, but it did "split the difference" in sound...one Elna was too noisy, the other Kaisei was too intense.

I can see a stuffy sounding vintage cap with poor high frequency transparency being bypassed by one that does to create some layering (?).

But that could create some weird situations - imagine caps which image poorly with a very mono sound, being bypassed in the high freq with a high fidelity cap. So the high frequencies image 3d dimensionally on top of an old low fi midrange? Or is that effect the goal? It could be interesting....??