Theoretical versus empirical equations
As pointed out already, the van der Waals (vdW) equation is also an empirical equation.  However, I think what you are saying is that the vdW equation is a physically motivated empirical equations where the empirical parameters in principle correspond to something "real" (e.g. molecular size and interaction strength) in contrast to a fit to an arbitrarily chosen fitting function such as a polynomial.

If you have lots of data points so that you can fit many parameters then a simple fit will be more accurate since it tends to have more fitting parameters than the physically motivated equation.  However, the simple fit will likely only be accurate for interpolation, and is still likely to be comparatively less accurate for extrapolation.