Simple resistance and simple impedance are not the same because impedance has a phase component to it.
For the sake of thinking about speaker impedance and amplifiers you can simplify it to resistance.
Speakers however are complex, and the impedance varies by frequency. Take a look at any Stereophile speaker measurement. The phase angle also matters as high phase angles vs. low tend to stress amplifiers more.
Most amps can handle most speakers, but knowing that a speaker has low impedance in certain areas can explain and help guide matching of a speaker to an amp. Not something you care about with normal, commonly available speakers.
It's the exotic speakers and too often, high end speakers, which have impedance issues to be aware of.