The reason this seems confusing is that you have just one sentence that is out of context. Yes, the continues tense emphasises the duration of something, but relative to what? It is relative to the context, and with no context you don't see any difference.
Peter went for a walk yesterday. He climbed up Balwins Hill and flew his kite from the ridge at the top. Then he built a den from sticks in the woods and pretended to be a wildman. He got really muddy before walking home.
So you see "flew a kite" is just one event that occurred during an eventful morning.
Peter has a kite. He was flying his kite yesterday when a hawk came out of the sky and attacked it.
So now you see that "flying his kite" was ongoing when another event occurred.
Long story short. Nobody ever says "Peter flew his kite" or "Peter was flying his kite" without context. And it is relative to that context that the choice of "flew" or "was flying" is made.