In human terms, I guess it's the same as middle aged ladies (usually) and their potential in getting varicose veins.
Those who spend their time on their feet but not moving about much (hair dresser, person at a standing till, etc) are not producing the pumping action that helps the lymphatic system drain properly (effective as from 7 steps or walking paces). These people are recommended to wear medical stickings/tights which compresses the veins and improves blood flow and hence drainage.
If horse has to be in all night, the stable bandages are the equivalent of the medical stockings.
Horses that are out are much less likely to have swelling problems but for many; keeping a horse out 24/7 does not fit in with particular lifestyles (especially if you don't have enough land to be able to do so, for example) :smile: