Studying life span development is important for many reasons. If we know about the cognitive, psychosocial, and biological processes of development it will help us understand human growth and development. The processes of development help us figure out about our health too.
It's also important to learn, because it teaches about risk factors affecting fetuses and genetic characteristics. We learn about health habits affecting longevity and how our genetic make-up plays a roll in our health. Knowing life span development helps us learn about mental and physical health issues.
Check out the link below for more information. The following quote is from the link below.
"Adaptation throughout life depends on how each of us negotiates the internal and external factors that enhance or constrain our abilities to reach our full potentials. The essence of life span development is in how we build our strengths to transcend these limitations over time."
It is important to study life span development because humans are living longer than ever before, and it is important for psychologists to understand every phase of human growth and development. While psychologists used to focus mainly on childhood as the time of greatest change in a human's life, psychologists now understand that humans continue to grow and change during each phase of their lives. The study of lifespan development emphasizes the idea of plasticity--the concept that people can change during any phase of their lives, and the field gives greater expression to the idea that human growth and development are not limited by time. In addition, the study of lifespan development looks at different factors, social, mental, physical, and emotional, that can affect people at each phase of their lives. This emphasis is important because it helps researchers understand how people can continue to live meaningful lives into older ages and helps researchers understand the multiple factors that affect people at each stage of their lives.
Like the previous answer stated, there are many reasons to study the lifespan of a people. Reflection will yield more reasons than can be answered in a short post.
First, from an educational point of view, knowing the development of a child can help enormously in developing academic curricula. For example, there is no use in introducing abstract concepts at an age when children cannot grasp abstract concepts.
Second, from a medical point of view, especially in that area of prevention, learning the development of a person can be of great value. For example, if a common disease or ailment is common in men in their 40s, then checking men in their 40s for this disease can be an important preventive measure. This can not only save a person's health but also save many tax dollars.
Third, developmental insight also helps religious officials. If you think about the sacramental system of the Catholic church, it is based on the lifecycle of a person - birth to death.