Because the nervous system is the master system in your body, we focus our attention on the health of the spine being properly aligned. If the spine shifts out of its proper place, then adjustments are used to help bring the spine back into alignment so the nervous system doesn't have interruptions in communication between the brain and the body.
Many things can lead to spinal mis-alignments including...
A vertebra going out of place (otherwise called a mis-alignment) due to an accident or injury
Trigger points and tight back muscles that pull the vertebrae out of place
The entire spine mis-aligning due to poor posture
Osteoporosis or degenerative changes of the spine
An inflammatory response caused by a poor diet, chemical or emotional stress
So, the next time someone asks why you get adjusted you can tell them, "I get adjusted so my brain can talk to my body without interruptions."