Throughout history, scientists have been trying to figure out what the world is made of. The Greeks concluded that the smallest thing was an atom until we smashed two together and discovered subatomic particles, which are anything smaller than an atom. Even neutrons can be broken down smaller. What we currently think of as the smallest things in the universe are elementary particles. These consist of fermions–quarks and leptons–which deal with matter, and bosons, which deal with interactions. Elementary particles, however, are so small we cannot even look at them with a microscope. If we did, the Heisenberg uncertainty principle would mean that we altered their place in spacetime. Consequently, when determining how they look, we plot a zero-dimensional point in space. This representation of elementary particles is called the standard model.
The standard model explains 3 of the 4 fundamental forces, Strong, Weak and Electromagnetic, but when it comes to gravity, there are complications. In Einstein's theory of gravity, you would need a boson called a graviton, but in the standard model, when accounting for a graviton mathematically there are infinities. However, when you apply it in string theory, the idea that elementary particles are made of strings that vibrate differently to determine the type of elementary particle, you can easily integrate gravity with the other fundamental forces.
String theory was invented in the summer of 1968 to account for the peculiarities of hadron behavior. In some particle-accelerator experiments, physicists realized that a hadron's angular momentum was exactly square to the proportion of its energy. The standard model of the hadron could not explain this. Thus, instead of the model that used smaller particles held together by a spring-like force, a model was made where each hadron was a rotating string moving in accordance with Einstein's special theory of relativity. This was the basis of bosonic string theory.
Quantum theory was discovered in the 1920s by Niels Bohr, Werner Heisenberg, Erwin Schrödinger and others. An experiment was conducted called the double slit experiment, where photons were shot through two slits to see where they would end up. You could see that the photons were in two places at once, however, when measured, it was 1 definitive place where the photon was. This developed the idea that something is only true once we have measured and observed it. Put in another way, it tells us that no two electrons, in the same system, can occupy the same energy state, and that all the energy states are filled from the lowest levels to the highest levels. A thought experiment that exemplifies this is ‘Schrödinger's Cat’. It says that if we put a cat in a box with a vial of poison that could explode any time and we shut the box, the cat is simultaneously dead and alive until we open the box and see for ourselves. However, when it comes to Einstein's theory of general relativity, which talks about light's relationship to time, you cannot have an electron or photon in more than one place at once, so string theory tries to explain how they can work alongside each other.
Supersymmetry is a scientific theory that says when elementary particles such as photons, electrons, and quarks were formed at the beginning of the universe, matching kinds of theoretical "super particles" were also created. If this theory is true, it would at least double the kinds of particles in the universe.
Dark matter is composed of particles that do not absorb, reflect, or emit light, so they cannot be detected by observing electromagnetic radiation. Dark matter is material that cannot be seen directly. We know that dark matter exists because of the effect it has on objects that we can observe directly but our knowledge is very limited about it. String theory is expected to provide a candidate for dark matter particles that works.
What Benefits can we already see in string theory?
Employment for string theorists
Gives rise to focused mathematical research that may aid in non-string theory applications
Helps physicists to evaluate what solutions to unsolved questions in fundamental physics might look like considering what we do know about string theory
To actually answer the question:
String theory has a way to integrate gravity with other fundamental forces as well as integrate it with the standard model.
It should provide ways to calculate the physical constants of the standard model and general relativity
It can help explain hadron behavior
It can help make a link between quantum physics and general relativity
Might reveal new physics beyond the standard model particularly physics that is important at extremely high energies such as those present in the first moments after the Big Bang.
It is expected to reveal that supersymmetry exists at a large scale
It is expected to reveal the properties of a neutrino
to provide a candidate for dark matter particles that actually works
to provide a mechanism by which the baryonic matter-antimatter asymmetry of the universe arose
It would rule out any hypothetical theories of physics that are inconsistent with it