As mentioned before, some music are recorded in the wrong phase so knowing the phase of your system allows you make the correct assessment (and adjustments) when playing these tracks. The music you are playing may well be recorded, or mixed with incorrectly setup systems.
So while your system is setup correctly at 0°, the music is recorded, mixed at 180°. When playing these music,you are still playing them with the wrong phase. If some tracks don’t sound good to you, invert the phase and see how that goes.
As always, listen to music by listening to the music. Do not over analyse. Simply sit back, relax and let the music (and the emotions) take you for a ride. Over analyse, and you become this guy below - completely missing the point of a hi fi setup.
So there you have it. Phase demystified I hope. Setup your system to be in phase, using the method you’re most comfortable with. If you are interested to use your ears to test for inverted phase, refer to this forum thread for a list of tracks that may1 be out of phase (link).
1 May because the verdict is still open if these tracks are truly out of phase, or not.