Same motivation as Galilean invariance, but relativistic version of that: we want the laws of physics to have the same form on all inertial frames, so we really want to write them in a way that is Lorentz covariant.
This is just the relativistic version of that which takes the Lorentz transformation into account instead of just the old Galilean transformation.
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Lorentz covariance is a fundamental principle in the theory of relativity that describes how the laws of physics remain invariant under Lorentz transformations, which relate the coordinates of events as observed in different inertial reference frames moving at constant velocities relative to each other. In more detail, Lorentz transformations include combinations of rotations and boosts (changes in velocity) that preserve the spacetime interval between events.