In my opinion, plate tectonics is most important for the study of seismicity, prediction of earthquakes and construction of earthquake resistant structures.
Plate tectonics is a geological theory of division of earth's crust into a set of tectonic plates that are in motion. This motion gives rise to earthquakes, which result in loss of property and lives. A deep study of plate tectonics can enhance our understanding of seismics (science of earthquakes) and help us predict future earthquake events. By knowing the potential level of an earthquake that can strike a given area, we can modify our building designs accordingly. For example, India is divided into a number of imaginary zones according to the threat of an earthquake. An ability to predict earthquakes can help us save lives and property. Sometimes earthquakes can also trigger massive tsunamis and hence the prediction of such events can help us save lives.
There can be a number of other applications of plate tectonics as well, such as distribution of resources and diversity of life, geomorphology, etc.