what it means to say maxwell's equations have 'predictive power' is that we can
1. take a situation observed or designed in the contingent universe,
2. translate it into the abstract entities maxwell's equations talk about,
3. deduce consequences in the world of abstract entities,
4. translate those consequences back into the contingent universe, and then
5. find that the consequences in the contingent universe are within narrow uncertainty bounds we translated from the abstract world of ideas
the meaning of turing universality is precisely that any turing-complete programmable system can be used to model any other logical or mathematical system, including other turing-complete systems, in exactly the same way that maxwell's equations model electromagnetism
for example, you can model risc-v execution in lisp and predict what a risc-v processor will do, you can model lisp execution in the λ-calculus and predict what a lisp interpreter will do, you can model the λ-calculus in a turing machine and predict what λ-reduction will do, and you can model a turing machine in a risc-v processor and predict what the turing machine will do
there is a significant sense in which this sort of modeling is much more perfect than the kind done with maxwell's equations
when we apply maxwell's equations, we are subject to measurement error in steps 1 and 5; our measurements are never complete and correct, and heisenberg's uncertainty principle strongly suggests that they never can be. and in step 3, because maxwell's equations are continuous-time continuous-space differential equations, we often also introduce numerical error in our calculations as well, because we usually have to integrate them numerically rather than algebraically
on the other hand, in the case of computational universality all the entities being discussed are discrete, algebraic, mathematically abstract entities, so our simulations are absolutely perfect unless we run out of memory or suffer a rare hardware error
obviously these universal machines are not limited to modeling other universal machines; we can also use lisp or turing machines or risc-v processors to model things like gravitation, taxation, or maxwell's equations. and they are obviously also the main working tool for all electrical engineers today, having displaced slide rules and load lines generations ago
ultimately, though, we are also using maxwell's equations (and other equations describing electromagnetism, like the ebers-moll transistor model) to design our electronic computers which we use to simulate lisp