The 20 amp circuit allows more current to flow than a 15 amp before the breaker trips, hence the recommendations to use 20 amp for your stereo wiring. Things like amplifiers can have transient power demands that a 15 amp circuit might 'stifle', the 20 amp gives a little more headroom.
I am probably using the wrong terms, but I hope you get the idea.
If only Almarg were here....
Bob