Although our bodies can bend and twist in all kinds of ways, they're designed to maintain a specific posture when not in action mode. Just like a marionette stands perfectly straight when the central head string is taut, so too should we stand as if a string were holding us up from the crowns of our heads, aligning our ears with our anklebones and creating a straight line that passes through our shoulders, hips and knees. By maintaining good posture, your muscles and skeletal system work together to put less strain on your joints.
This takes some effort and thought, as slumping or slouching is the normal stance for most of us. But good posture limits your bones from rubbing against each other unnecessarily at the joints, preventing or putting off the appearance of arthritis. Once you're used to standing ramrod straight, your muscles will feel an overall ease in stress, since they won't be in constant use to maintain an unhealthy zigzag body form.
Your back and neck will especially thank you, and good posture will help your shoulders, hips and knees from feeling tight.