Your spine houses your nervous system - the most delicate and important organ system - responsible for your day to day bodily functions. From your immune, cardiovascular, and digestive system, to sexual function and mobility - your nerves are responsible for sending vital information and energy from the brain to your organs through your spine.
Just as a kink in a garden hose can dramatically reduce the flow of water coming from the tap - a spine out of alignment dramatically reduces vital information going from your brain to vital organs.
Thus, your orgrans are much more susceptible to malfunction and disease.
It's not just coincidence that most people with poor posture tend to lack energy and suffer from fatigue, experience random aches and pains, as well as suffer from headaches/migraines, allergies, ear infection, sleeping disorders, gastrointestinal
issues (such as GERD), asthma, high blood pressure, etc.