The main reasons that I invest much of my practice time on standing Qigong are as follows:
Standing gives you awareness of your body and Qi.
Being able be feel exactly what is going on in and around your body is a prerequisite for making improvements. A teacher can show you something, but until you can feel it happening in your own body then its just interesting information (having the plans to build a house does not mean that you are capable of building it).
Standing Qigong gets fast results
When you're standing still there is nothing to distract you from what you need to work on. Your weaknesses are laid out on the table in front of you and you've just got to deal with them. This allows you to build a strong foundation from which to progress into moving forms (Tai Chi etc.). If you just focus on physical movements, it generally takes alot more time to build strong connections within your body, it is actually more time efficient to do this standing than moving.
You can practise Standing Qigong anywhere
I prefer to practise outside whenever possible. However, for several months of the year there is little or no daylight outside of my work hours. Being able to still do my practise inside when its dark, cold or raining helps me to keep up a regular practice.
The Qualities you gain whilst Standing are transferable
Whether you train one standing posture or another, the skills you gain can be used anywhere in your life; any Martial Art, internal or external will be improved, sports you practise, your health and strength will improve, and your concentration.
Your Breath can also be trained while doing Standing Qigong, and there is nothing you can do whilst alive that will not benefit from better breathing, including all forms of movement arts, sports, studying and mental work, Meditation, Sex and of course sleeping.