Why does your body remember what your mind forgets?

It sounds strange the body can remember things the conscious mind can’t remember, but its true. What does this even mean? Why is it useful to know?

The other day while swimming in the local lake, I hear raised voices just out of sight. In the first few seconds I’m not sure if it’s excitement or aggression. My shoulders and neck tense up. My nervous system doesn’t wait for clarity, it goes straight to its recollection of what raised voices might mean (danger), and I who I am/was that moment -- not here and now where I am actually safe.  

Our survival system is so well designed to keep us safe, that it also remembers when we were not. Just in case. But sometimes it does its job so well, that its old response is so strong, that it shifts our awareness out of the present to what is really it’s memory of the past.