Why focus below your bellybutton in meditation? My roshi tells me to focus here. Before practicing, I spent most of my waking hours thinking, analyzing, living in my head. With practice, it has gotten easier to think when thought has a purpose, and to listen to my body and environment otherwise. 

The book that introduced me to zen said to picture your breathing below your bellybutton. A friend said to focus your energy there.

As I build focus here, it is easier to let go of distraction and delusion. But why is this place significant? Why not focus on my chest of elbow or toes?
Many types of meditation focus on the breath entering and leaving the body or moving through the body as a main object of meditation.
In the tradition of meditation which I practice, (Mahasi Sayadaw style), we focus on the rising and falling of the abdomen as a way of being mindful of the breath, but without focusing on the nostril area, which is considered to be more soothing. Focusing on the rising and fall of the abdomen is not soothing and works well as a meditation object which will keep us mindful of the three characteristics of impermanence, dissatisfaction, and uncontrollability/anatta which is useful in Vipassana meditation.
But you mentioned zen, so your roshi's reason may be quite different and it would likely be best to ask your roshi directly. But if you cannot, here is a quote from Zen Master Thich Nhat Hanh's book &quot;be free where you are&quot;.
(In this chapter Thich Nhat Hanh is talking to prisoners about how to handle feeling overwhelmed by a strong emotion.)

So when you notice that an emotion is beginning to come up, it is very important that you put yourself in a stable sitting position, or you lie down, which is also a very stable position. Then focus your attention on your belly. Your head is like the top of a tree in a storm. I would not stay there. Bring your attention down to the trunk of the tree, where there is stability.
When you have focused on your belly, bring your attention down to the level just below the naval and begin to practice mindful breathing. Breathing in and breathing out deeply, be aware of the rise and fall of the abdomen. After practicing like this for 10, 15, or 20 minutes you will see that you are strong - strong enough to withstand the storm. (page 38)

So while both styles of meditation focus on the abdomen, the reason is a quite different. In one, the rise and fall of the abdomen is considered somewhat unstable as we notice slight irregularities in breathing. In the other, the abdomen is considered the stable part of the body as a tree trunk is to a tree.
Due to the many differences in interpretation, best to ask your own teacher if possible. But something to think about until you get the chance.
Welcome to Buddhism.SE and glad to hear of the benefits meditation is bringing to you. :)