Yes, it is good for their coat.
Spin the egg! If the egg is raw it will wobble, because it's still liquid inside, if it's hard boiled it will spin on it's end, because it's solid.
Soft-boiled eggs are cooked for around 3 -5 minutes while hard-boiled eggs are cooked for around 7-8 minutes.
Soaking in cold water also stops the black ring that forms round the yolk if you let it cool naturally
Diffusion between hypo/hypertonic substances across a semipermeable membrane. AKA: The water from the vinegar fills the egg