The study of geology is important for three main reasons: it reveals the deep history of the Earth, informs other sciences, and it is useful for economic purposes. Before geology was a science, the age and development of the Earth were mysterious, and prospecting for minerals was a hit-and-miss business.
An early objection to the theory of evolution was that of Lord Kelvin, a physicist who claimed the Earth could not be old enough to allow time for the development of complex life. It was the study of geology, and the subsequent discovery of deep time, that demonstrated that Earth has had billions of years to evolve life. One branch of geology, palaeontology, even turned up fossils that support the biologists' claims about evolution.
In addition to its academic usefulness, geology also informs commercial efforts to extract oil, gas and ore. Oil companies have to pay for every hole they drill, and anything that increases the chance of striking oil saves money. A geologist can determine, from the types of rock in the area and the so-called "indicator fossils" present, whether a drill site is likely to have oil under it. Some geologists, called gemologists, have helped mining companies find diamonds and other valuable gemstones.