Why Dynamic Drivers for Speakers/Headphones/IEMs?

Was watching the following podcast, and Brent B. mentioned another difference between dynamic and planar mag headphones I’d forgotten. Planar headphones tend to have a flat impedance response, whereas the impedance in most dynamic headphones tends to vary somewhat with frequency. This matters because the headphone’s impedance curve interacts with frequency, expecially when using a higher impedance amp. The video below is cued to this question.