The value of having all speakers tested identically is that one can accurately evaluate the RELATIVE efficiencies of two pairs (or more) of different speakers. Regardless of what the impedance does, regardless of what room they're in, a speaker with an anechoic sensitivity higher by 3 dB or more than another indicates that (ultimately) the higher sensitivity speaker takes fewer watts to drive.

Depending on how loudly you want to listen, depending on how large your room is, and depending on how much amplifier power you can afford, knowing the speakers' sensitivity is a valuable thing.

It would be impossible to compare speakers efficiencies were not the test conditions standardized.

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