Science knowledge can be defined as an understanding of how and why certain science phenomena occur in nature. Usually, growth in science knowledge is initiated when we observe an occurrence, question why or what is happening, and hypothesize on how it happens.
Human Curiosity: The "why" factor. Why does something happen? This curiosity is what led to the understanding of gravity. For example, Issac Newton is probably most famous for being curious as to why an object always seems to fall to the ground. This motivated him to try and reason as to why this is. Eventually, this led to the concept of gravity, a force of attraction towards a mass due to its bending of space. Hence, our scientific knowledge is enhanced due to a better understanding of one of the greatest forces of nature.
Human Need: The "how" factor. How can we learn more about a subject to solve this problem. The most recent example is Obama's initiative to map the brain. By mapping the brain, we can understand how certain disorders, such as Alzheimers, autism, Parkinson's, etc., arise. By understanding how they arise, we hope to be able to come up with a solution to prevent or treat their occurence. Needing to improve health and prevent such disorders from happening, we look for ways to increase our scientific knowledge and understand how they work so we battle it.