I don't know what anyone can add that you can't find in the manuals. Simply put a power conditioner (vs. a simple filter) regulates the voltage to a "constant", say 117 volts, here in the US, or, say, 208 volts there in Germany that can be harmful to equipment, including surges, spikes and brownouts. Typically not necessary unless:
a) you have some high end equipmment; and
b) your electrical supply is unstable.
Otherwise, a simple (or a stage four) filter/suppressor is fine in the vast majority of cases. At least that's my take on it. Cheers.