Basically they improve blood circulation and prevent varicose vein problems.
My old coach was convinced it improved recovery time for lower-leg muscles after hard training and more or less made us swear to use them as much as possible :)
>It's funny, after you do it long enough your body craves it.
I'm doing daily (well at least 5-6 days a week) walks myself as well, I do between 50-70 minutes depending on how I feel, and yes it does become something of a craving, I typically feel uneasy if I don't get out and walk.
The reason I started was as part of rehabilitation after a knee injury I sustained during sports, and it became a great habit (something good came out of that injury).
This might also play in to avoiding vein issues when standing since you get the leg circulation up and running during high pace walks.