Load flow is implemented so that users can start a simulation from a steady-state condition. It is an analysis to determine the voltages, currents, real and reactive power flows under a given operating condition. Given the system admittance matrix, 4 variables (P, Q, V, theta) are associated with each bus to solve two power flow equations. Therefore, for each node, at least 2 out of the 4 variables should be known to solve the equations.
For background, see IEEE Std 3002.2™-2018 Recommended Practice for Conducting Load-Flow Studies and Analysis of Industrial and Commercial Power Systems.