Why it matters.
It gives different camera controls a common scale.
A closer look.
Doubling shutter time adds one stop of light exposure. Aperture steps such as f/4 to f/2.8 also add about one stop. Doubling ISO adds one stop of output response, without adding light.
Picture this
Change 1/250 second to 1/125 second, then f/4 to f/5.6. The two light changes approximately cancel.
Keep in mind.
Camera displays round aperture numbers; fractional stops are common. ISO stops describe response rather than light entering the lens.