When measuring distance, you can use a variety of units: inches, centimeters, miles, fathoms, etc. Some units are larger than others. Some are more familiar than others. However, regardless of the units you use, the distance you measured remains constant but the number you get can vary widely.
Similarly, when measuring radiofrequency signal strength, many different units can be used. They are all equally valid, and you can convert from one unit to another. But some units are much bigger than others.
You wouldn’t measure the distance from Paris to Rome in inches; you would use a bigger unit… perhaps kilometers. Likewise, you would use a very small unit to measure the thickness of a human hair. So, when measuring the generally low levels of background RF radiation, you can use a small unit, such as V/m. When measuring the relatively large amount of radiation that leaks from a microwave oven, you can use a larger unit, such as mW/cm2.
The image shows the relative size of some units compared to 1 V/m. Notice that A/m is a much larger unit, almost 400 times the size of one V/m. A/m is better suited for measuring very strong radiation.
* Engineers will have a preference for one unit over another, particularly when measuring in the near field.