Quantum field theory underlies all of quantum mechanics; it's what you have to use to study the elementary particles of nature, and it's uses are enormous. Currently, three of the four forces, the strong, weak, and electromagnetic, are described using quantum field theories that are typically lumped together into the Standard Model of particle physics, and the Standard Model is used for calculating collisional cross-sections of particles, for astrophysics (including Big Bang cosmology), for corrections to Schrodinger atomic physics, and all sorts of other things. The predictions of the quantum field theories have been among the most accurate of any theories in physics, too. So that's why I've been studying field theories this summer.