Here's and interesting little fact that one Northeast teacher once told me - chickens can sit a long time! It doesn't necessarily mean they are very wise. There are four main postures: sitting, standing, walking and lying down. We are in these postures throughout the day. If we don't get practice when we are in these postures, then we tend to view meditation as something we do just when we sit. Then we don't integrate our practice into our normal life.
Walking meditation can be very helpful for developing our awareness throughout the day, developing more energy in the mind, which is a factor of enlightenment. We learn how to focus the mind while the body is moving. Sometimes if we just sit and we don't get this practice, then it's difficult to be able to be mindful while we are moving. Sometimes people just want to sit because they get attached to the tranquility of just sitting. As well, if we sat all the time it would get very painful, so it's very helpful to move the body not only for the health of the body but also for the health of the mind.