MALT assumes that the potential at all points of the ground grid is the same. MALZ provides for voltage drops in the grid conductors. There can be a difference in large grids if the soil resistivity is low, but normally MALT is adequate for normal step- and toudh-voltage calculations. If there is a difference in grid potential, then the location of the fault is important, so it complicates the analysis.
MALZ also is needed to analyze high frequencies such as lightning.