Answer:
Science is a vast subject. Scientists have classified it into different areas/branches so that they do not get their concepts mixed or overlapped. We can think of all the sub-branches as different topics studied under one main subject, science. We just divide the main subject so that we can study each of the topic clearly and individually. Science is such a vast subject that most of its sub branches are not even related to each other or interlinked, like computer science and biology. If we did not have subdivisions in science, can you imagine a person studying biology, computer science, chemistry, physics, phycology etc all at the same time, simultaneously? Of course not! That's why, in order to get a clear understanding of each topic, we divide science into different areas.
Hope that helps...