why knowing what you believe (foundationally) is important

Well, it's important for all sorts of reasons, of course, not least of which is the simple matter of truth. But in terms of inter-personal & organisational dynamics, Fried & Hansson put their finger on it:

When you don't know what you believe, everything becomes an argument. Everything is debatable. But when you stand for something, decisions are obvious.

(Fried & Hansson, Rework, p.44)