If you ask a prospective candidate seeking admission into a university or polytechnic to study Geology that "why do you think Economics as a subject is important for you"? Perhaps, he would tell you Geology is an applied science and has nothing to do with Economics, a social science subject.
In the long run, after the first year which is a preamble year into the study of Geology and he is to take courses in Economics aspect of Geology; he would probably remember your question.
Economic-Geology is the branch of Geology that examines the economic parameters which determine if a proposed geological project is viable or not.
This is the major aspect of geological considerations which oil exploration and solid minerals exploration companies don't joke with. However, it is irrelevant to a layman outside the profession. This is why you hear laymen say: "we have petroleum deposit in my community, but is not tapped; we have gold deposit in my town but is not exploited".
The salient explanations to those scenarios is owed to Economic-Geology. This is where we apply the principle of the "Return on Investment"(ROI) to determine if the project is viable or not.
This is simple. After all the favorable and unfavorable factors regarding the project have been considered, vis-à-vis the fund to be sunk in the project compared to the revenue to be generated from the project and lastly the lifespan of the project, the company can now decide whether to continue with the project or not.
Hence, the knowledge of Economics is vital for Geologists so as not to sink the whole organization into bankruptcy.