A lot of Theoretical Physics and String "Theory" in particular is pure speculation. They are using the word theory is a non-scientific sense and popularized their notions with books for the general public. It's actually sad what they've done.
Did you know that General Relativity and Quantum Mechanics started out being "theoretical physics"?
They became more than theoretical, once they have found some evidences to back it up and some applications.
Without General Relativity, they wouldn't understand modern astronomy, stars and galaxies, or the physical cosmology, like the Big Bang.
General Relativity have also given us better understanding of gravity.
While Newton's law on gravity and motion are useful for on Earth and with objects moving lot slower than the speed of light, his theory on gravity was incomplete, limited and not very accurate in space.
Albert Einstein didn't completely replace Newton's law on gravity and motion, he did extend beyond the old theory.
Quantum Mechanics (QM) have given us better understanding in the older electromagnetism, thermodynamics, field theory, nuclear physics and study of atoms.
In particle physics, we have managed to identify particles even smaller than protons, neutrons and electrons, like quarks, leptons, photons, Higgs boson, etc.
Without QM, we wouldn't understand nuclear fission and nuclear fusion. Nuclear fusion gave us a better understanding of how stars work, eg Stellar Nucleosynthesis.
Nucleosynthesis is the fusion of two (or more) lighter atoms into heavier atoms. For example, the sun would fuse two hydrogen atoms into one helium atom.
Another nucleosynthesis was written about by George Gamow in 1948 - the Big Bang Nucleosynthesis (BBN) - wrote how matter first formed, by combining protons and neutrons within the nucleus of an atom.
BBN started not long after the Big Bang, about 3 minutes after the universe began expanding from its singularity. In the next 17 minutes, ionised hydrogen and helium atoms formed - ionised atoms are atoms without electrons.
Electrons did bond with atom nuclei until the Recombination epoch (started 377,000 years after the Big Bang), when the atoms became more stable and electrically neutral. We are only be able to observe at this period, the Recombination epoch.
Although BBN is still theoretical, it is accepted by every astrophysicists and astronomers, including cosmologists.
Theoretical physics can become experimental or empirical physics, once evidences and applications are found.