How to make sense of EIRP?
If the energy is indeed focused, either by a dish parabolic or some yagi,
then the main lobe will have high energy density, but as you model (or measure) slightly off bore-sight, the energy density will be much much lower.
In your example --- 79 watts/square meter --- the peak energy density will only exist for +-10 or +- 20 degrees around bore-sight, in both horizontal and in vertical axes.
At one meter distance, +- 10 degrees is only  +- 15 centimeters; for both X and Y, the area is ( (2 * 15) * (2 * 15))  or 900 cm^2, compared to 10,000 cm^2 per square meter.
Where does this energy come from? from starving the other (360 - (2 * 10 )) or
340 degrees, in each axis.
Those other regions will be 0.1 watt/meter^2 or 0.01 watt/meter^2.
In general, the weaker the response of the off-bore-sight, the better. Especially for radar systems, that work with Range^4 issues, the side_lobes permit jammers to become a problem.