Metaphysics and epistemology map out the terrain for the long walk
Metaphysics and epistemology are useful in informing how I can live because they are the only tools that could get me to what is true. Knowing what is true is useful because human beings have been thinking for a long, long time that theories abound about how we should live abound to the point where the terrain is very muddled.
The question that metaphysics answers (What exists?) and epistemology answers (How do I know what I know?) are like a compass. The answers I get from contemplating them allow me to see the terrain and create a map out of it. When the terrain is mapped, it is easier for me to answer the question that I am most interested in: how to live?