Why Measuring an actual RF field means quantifying volts per meter (or power density) in the air, and that requires an instrument whose response is traceable to a known standard. A field strength meter reads the voltage its antenna picks up, so the antenna's calibration factor (gain and effective length versus frequency) must also be known to convert that reading into field strength. Without both the meter and the antenna being calibrated, you only get a relative indication, not an accurate number for a routine RF exposure evaluation.
Watch out An SWR meter only compares forward and reflected power in the feedline, which tells you about the match at the transmitter, not the strength of the field radiated out where people might be standing. A receiver with DSP or a scope with a marker generator can show relative signal levels or frequency accuracy, but neither is calibrated in volts per meter.
Field strength is measured in volts per meter, so you need a calibrated meter AND a calibrated antenna, not a feedline gadget.