A phased array is capable of "beamforming", that is, sending an electromagnetic signal most strongly in a specific, programmable direction, as opposed to broadcasting the signal in many or all directions.

EIRP is a measure of the maximum power in any direction, so a phased array that transmits 1W only forward has the same EIRP as an omnidirectional antenna that transmits 1W forward, but also 1W backward, and up, and down, etc. Overall the omnidirectional antenna may transmit much more power total, but still only 1W in any particular direction.