The practical part of your question is answered nicely by John Forkosh. There is no way we could at present guess at the practical applications of a theory of everything, but previous experience more or less projects that there will be, inevitably.
I will address this:
benefit in explanatory value of studying something (like fluid dynamics) at increasingly more basic/marginal levels?
similar to what Faraday said to queen Victoria:
what is the benefit of a painting by el Greco, or any of the masters?
what is the benefit of James Joyce's literature?
etc., wanting to point out that human nature contains creativity. Once a physicist has the mathematical tools, in addition to logic, creativity takes over for the few gifted ones, and it is as inevitable that they will use the tools to express their physics intuitions, as it was that el Greco would paint. Very few reach that level, similar to there are millions of painters but few that really are acknowledged as masters. The rest of us physicists, are lucky that they appear, as Feynman did and gave a great push forward in calculations.
Creativity is driven by intuition and a sense of beauty, and "rightness", and the more involved theories of physics are beautiful, for those that can understand the mathematics, even though not art, simplicity being a driving force. That is why the theory of everything is attractive as a goal, and all unification theories.
of studying something (like fluid dynamics) at increasingly more basic/marginal levels?
Physical theories have many mathematical frameworks, and it is satisfactory to be able to prove that at the interface there is complete understanding. Understanding is another human trait, and one wants to know how the various frameworks fit the same observations. In chasing for this goal, new effects can be found, look at transistors and other quantum mechanical applications, once quantum mechanics was understood.