If you live or gig near a step-down trafo-station that's also feed a factory with old equipment You would be amazed by how much overlaid DC, high values of harmonic noice you can measure op top of the "clean" A/C voltage.
DC voltages between what and what? Voltages must always exist between two points. DC voltages, as described, are not problematic for electronics. But can be a serious human safety issue. A threat that would not exist if the utility and a building's electrical system is properly earthed. Not just any ground. A unique ground defined by earthing electrodes.
DC voltages cannot pass through utility transformers.
Harmonics also are made irrelevant by circuits (as another described) that make DC voltages irrelevant. Harmonics can be a greater threat to motorized appliances.
Meanwhile, harmonics and many other anomalies are made irrelevant by how electronics get and regulate power. No matter how 'clean' or 'dirty', a power supply converts that into a much higher voltage (exceeding 300 volts). Then converts that into radio frequency spikes (spikes are rich in harmonics), If harmonics are a problem, then the worst harmonics are created by each electronics' power supply.
Since best filtering is already inside every supply, then high voltage radio wave spikes are converted to rock solid and stable, low voltage DC. For many anomalies, it does not matter how clean power is. Since most electronics convert 'clean' or 'dirty' power into some of the 'dirtiest' in a building. Then superior filters inside each supply 'clean' that 'dirtiest' electricity into 'cleanest' DC voltages.
Some anomalies can corrupt that superior filtering. A most common path is one that bypasses superior supply filtering.
To say more requires discussing each anomaly uniquely. IOW no standard defines line conditioners. The term is quite subjective - which advertisers love because myths are then easily generated. Solutions do target unique anomalies. One useful solution might address noise. But to be sufficiently sized, these series mode filters would be tens of kilograms. Many (ie Furman) claim to eliminate noise. But when we read its numbers, that subjective 'cleaning' is near zero. Just large enough to claim total filtering - subjectively.
Important for each anomaly are specification numbers. For example, all electronics do superior line voltage regulation. Axe-FX spec numbers say it is even better.
Will a $200 Furman protect from lightning? It does not even claim to. But superior solutions for $50 or $80 do protect from lightning. Protects everything including the Furman. And remains functional after each strike. An example of an effective solution unique to one anomaly that also costs less money. With manufacturer spec numbers that say how effective it can be.
