Just imagine not studying it. The roadmap to our existence would never be known!
Life is more than just lungs that breathe in oxygen and a heart that pumps. It is a composite of multiple processes that affect a living creature, both mentally, physically, and even spiritually (although the latter is not part of the study of lifespan development).
Lifespan development involves the analysis of activities, patterns, and all interactions that take place within the systems and subsystems of the human body. From the cognitive, to the psychosocial, to the biological, it is good to know what humans "are made of;" it helps us predict behaviors, find interventions to common developmental issues, and understand what are the causative factors that bring one stage after the other.
The complexity of life itself is the primary reason why lifespan development studies deserve a front and center rank in everyone's lives. This is especially particular because life is finite, can end at any moment, and many factors endanger it. Knowing what comes before and after each stage of life is the best way to prepare for what is coming; it is also the best way to live life with dignity, self-care, and purpose.
Lastly, lifespan development studies apply to more than one field of study, from engineering (our bodies are just like engines of action and thought), to education (development is everything in learning), to psychology, medicine, and much more. Not knowing what takes place in this complex journey called "life" would be doing ourselves a huge disservice.
For more info on lifespan development and theories of development look for Jean Piaget, Erik Erikson, Sigmund Freud, and Lawrence Kohlberg.