Simple.
Your feet are meant to bend and move and flex and FEEL the world.
If you don't let them do their job, the function of balance and agility tries (unsuccessfully) to move "upstream" into your ankle, your knee, your hip, and your back.
A lightweight, minimalist boot that lets your foot do what's natural can take the stress off those upstream joints.
In other words, they let your feet do their job so the rest of your body can do its job.
Don't neglect the FEELING part -- you have more nerve endings in your soles than anywhere but your fingertips and lips. Your brain needs that feedback from your feet for proper, efficient, and enjoyable hiking and walking.
Xero Shoes' boots are made to let your feet do what's natural so you can Live Life Feet First!