Walking meditation is great to developp awareness in everyday life. The setting is more likely to reproduce what you will experience throughout your day.
While in sitting meditation, there are usually much less distractions occuring.
So, from my point of view of a relative beginner, sitting meditation is effective to empty your mind and experience higher levels of concentration, and walking meditation is effective to help you carry this concentration into your daily life.
Walking meditation is also very convenient when you lack energy, you will usually feel much less sleepy walking than sitting.
In addition, if you already seat all day at work and meditate for long periods of time, alternating between sitting and walking meditation might be a good idea to maintain a healthy life style.