Phase is simply put, Power Factor. Another measure of reactance.
The larger the phase angle, the less power delivered to the load. A 1000 watt amp trying to drive a load which has a 90` phase angle will deliver exactly ZERO power at that frequency. cosine 90=0
A speaker which is a difficult load would, IMO, have large phase angles at the same frequencies where impedance dips occur. A double whammy.
My panels? Not a bad load, at all. low impedance, to be sure, but no wacky phase angles to suck up amp power.