With a DC load line (DCLL), a buck regulator will essentially present itself as a fixed resistance to the output load.
In a circuit where there is a 150A load being pulled by an ASIC for example, at a nominal output voltage of 0.8 V, and the regulator has a DCLL of 0.5 mΩ, the output volatge will drop by 150 A × 0.5 mΩ, or 75 mV, to 0.725 V. This drops the power consumption of the processor by 11.25W (75mV x 150A), easing strain on whatever heatsink or thermal solution is in place. When the load current drops below 150A the output voltage will rise accordingly. Load lines make meeting the transient specifications much easier by reducing the amount of output capacitors needed.