The most compelling idea is that harmonic noise is caused by the loads in a facility, and it accumulates via the wiring. Ergo, the deterioration of the power quality is cumulative.
From an engineering perspective the niftiest feature of the filters is that they are passive and installed on the neutral. They use no power and suffer no wear or tear, plus, in the theoretical case that they were non-working, there would be no interference on the circuit, since they are in parallel and not in-line.
A simple way of looking at the benefits of harmonic filters is this: in the 60's and 70's the typical electrical loads in buildings were predominantly inductive, and had leading power factor, and we compensated for that with capacitors to absorb the initial power surge when equipment switched on.
Today, the most prevalent loads are capacitive in nature (LEDs, VFDs, switch-mode power supplies) with lagging power factor, and we compensate for that with an inductive transformer to kill the harmonic noise. The result is typically:
- a 10-25% reduction in electrical bills,
- lower maintenance costs,
- lower capital expenditure, and
- reduced risk of electrical fire.
The payback, based on the power savings alone is usually under 18 months, so it will average down project payback and magnify financial results from any given retrofit project.
Here is a link to an article that puts in perspective the power quality problems of today's energy efficient lighting.