I like this question. Perhaps the question should be; “Why do we search for a Unified Field Theory?” It’s best to ask this question to the man who started the search, Albert Einstein. Einstein spent the last thirty years of his life searching for a way to combine gravity and electromagnetism into a single elegant theory.
I believe that Einstein was motivated by an intellectual need to unify the forces of nature. I fully agree with that idea. To me it just makes good common sense that the forces of nature are slightly different manifestations of the same thing. Each force should be transformable to the other via a mechanical process. He felt very strongly that all of nature must be described by a single theory.
Einstein said this in his Nobel lecture in 1923, “The intellect seeking after an integrated theory cannot rest content with the assumption that there exist two distinct fields totally independent of each other by their nature.” Ironically if anyone ever succeeds in this quest it will most assuredly spell doom for the advocates of Quantum Mechanics.
Einstein always insisted that quantum mechanics could be derived from some more complete theory. For Einstein, who was never satisfied with the weirdness and randomness inherent in quantum theory, any acceptable unified field theory had to have quantum mechanics as a consequence. The Unified Field theory will subsume QM.
In the end, when the Unified Field Theory is discovered I believe it will lead to technologies that will revolutionize the world we live in. So in a very important way we do need this to happen. Solving the Unified Field Theory problem will allow us to discover the truth about our physical reality and also allow us to advance technologically.