Why do we need a Unified Theory of Everything?

We don't need them in the same way we seem to need other theories in physics, but the conjecture that the basis of the physical world can be represented in a small collection of simple ideas, seems to take on momentum as new discoveries steadily come in. The 'unification' of magnetism and electrostatics by James Clerk Maxwell was driven by the experimental discoveries in the early part of the 1800s. This isn't just an ad hoc idea in physics, but a statement of how the world actually operates when it comes to electricity and magnetism. In the 1960's, the electromagnetic and weak forces were 'unified' by Weinberg, Salam and Glashow. Their motivation was a combination of detective work and theoretical 'fancy' but they ultimately found a mathematical theory as Maxwell had, that described two forces in terms of a single more 'unified' description. Experimental evidence has since proved much of what they predicted to be correct. Currently, physicists are working on a still more comprehensive unification theory that combined gravity, and the strong force, with the electro-weak 'force'. We don't really know if that program will work because the experiments are frightfully hard to conduct.

We don't need a Unified Theory, but it seems that nature likes to work in this way at least some of the time. In part, it is human nature to search for simple answers, and Unification Theory is the ultimate 'simple answer' for explaining the diversity of particles and fields we now know about.