This single equation, however, will refer not to anything immediately observable but rather to inferred underlying structures. The observable features of the world, the features that our sense organs and measuring instruments happen to be well adapted to perceive, can, however, be elaborate combinations of these simple ingredients. The observable combinations may behave in complicated ways that require a lot of mathematical and logical work to derive, though ultimately they are consequences of the single equation of our deep theory. If we are to understand complicated appearances in terms of underlying simplicity, this is the price that must be paid. Depth is not at all the same as fruitfulness.
Depth is like the root system that supports the visible tree.
Einstein's theory of gravity, at the time it was formulated, made very few testable predictions that could not be derived, much more easily, from Newton's theory. In the long run, however, the deeper roots will support new growth. As a bonus, we hope and expect it will yield new fruit, as it always has in the past. A visitor to the gaslit halls of the London Exhibition of 1851 asked Michael Faraday what possible use could arise from experiments on electricity. This geometry can help I may reason logically thru theorem and propositions galore, but only what I perceive is real.