Science is a way of describing "why stuff happens." More specifically, through observation, empirical research, and experimentation, we have, over centuries, been able to more adequately pinpoint the cause of previously unexplainable phenomena.
Now, specific to your question, science is crucial for understanding how people develop. In this case, science helps answer "how and why we develop." Many scientific theories are influential to understanding human development, but for the time being, please consider the following...
Nature vs. Nurture: Do we become who we are because of our genes or because of our upbringing? (Modern science says a combination of both is influential).
Continuity vs. Discontinuity: Does human development occur gradually or does it happen through distinct stages?
Stability vs. Change: Do people's personalities remain relatively stable or can they change?
