Good question. Why would anyone study rocks? Well, if we probe a little deeper, there are some reasons why people should study rocks.
When people want to learn about the past, they are immediately handicapped, because the past does not always leave records of what it was like. So, scientist and historian need to use whatever there is. In some cases, the only thing to study is rocks. And if a person studies these rocks and compares these rocks with other rocks, there can be some great insights. For example, the study of rocks can tell you, about past temperature, how thick the atmosphere was at a given time, whether there was a body of water, or a volcano, or perhaps something else. Rocks can even tell you about past in greater detail, if things like fossils are on these rocks. You might say that rocks are clues to the past.