Why do Injera recipes call for adding boiling water or cooking down tef with some water?
I can’t give a definitive answer for Injera, but in baking, a boiled water/flour mixture (called tangzhong or  water roux) is added to increase the dough’s water content, effectively binding more water in the dough than a plain dough could. The results are typically very fluffy and light. 

I suspect that Injera with this water roux would turn out more pliable and a bit softer. Considering that you use bits of it to scoop up the stew that is usually served with, I would expect it to be more “bendable” and texture / mouthfeel would certainly also benefit.