Power factor (PF) is a topic that has been understood since the advent of electrical distribution, due in no small part to Faraday's experiments and Maxwell's subsequent mathematical expressions. It is defined as the ratio of real power flowing to the load to the apparent power in the circuit.
When you hit the brakes in your car, you get a linear slowing effect. Operating at low power factor is a little like having the brakes working on only one side of your car. Another analogy might be a large engine with an undersized fuel line.
The two primary reasons for power factor correction (PFC) are efficiency and signal quality. Operating at poor power factor means under-utilization of wiring, including the windings in the generator itself. Heavy loads/sources that are not PFC can distort the sine wave enough to confuse other equipment.
In an old fashioned transformer-based battery charger, you will typically find a choke that performs the PFC function. This technique is nice because a choke is ultra-reliable (because it's basically a one-winding transformer), but it can only be tuned to a single, fixed operating point. A charging battery's voltage changes, so by definition is a variable operating point - making the choke technique less than ideal.
Most of the leading brands of whole-house, battery-based solar PV inverter/chargers come equipped with a 60Hz transformer and solid-state control, so implementation of power factor correction requires little additional hardware (the transformer does double-duty, performing the function of the choke). Contrast this with designing an AC-source, DC-output power supply or battery charger from scratch, which demands specific intent and components to perform PFC.
Our Magnum MS series inverter/chargers feature PFC for energy savings of 25-30% when charging batteries.
Perfect power factor correction means that at any particular instant, you can look at the voltage on the sine wave, and by that measurement know exactly what the current is simply by dividing the source voltage by the resistance of your load. A purely resistive load like a heater is already PF 1 so it would not benefit from power factor correction. Converting AC to DC, however, requires diodes, which are inherently non-linear and will result in a power factor somewhere between 0 and 1.
To PFC or Not to PFC?
Inductive loads like motors cause a phase shift between current and voltage, reducing the power factor and therefore the transmission efficiency - but they do it in "nicer" way, which can be counteracted by the addition of a capacitor; their phase shifts are opposite, so they cancel. Diode rectifiers, however, pull amps when input voltage is above output (and they don't when it's not). That's like having a load that you're turning on and off 60 times a second. At every turn-on and turn-off you get a ripple that has RF components that affect other electronics, if even by mere proximity. Power supplies with rectifier front-ends (practically all) can benefit from PFC.
So why aren't all power supplies designed with a power factor correcting front end? Basically because PFC requires an additional set of components, which adds to complexity and cost.
It then becomes a question of whether the added cost results in enough energy savings to justify its implementation. The answer will be dependent on the utilization ratio of the supply in question. If it's used a lot, or if electricity is expensive, it's probably worth it to invest in PFC.