What is the use/importance of the equilibrium constant Kc
$K$ is the value of $Q$ at equilibrium. The expression for $Q$ and $K$ are the same however $Q$ describes the state of a system and $K$ describes the system at equilibrium. Le Chatelier's principle tells us that a system that has $Q\neq K$ will approach $K$. When $Q$ is not $K,$ the system is not at equilibrium.

K tells you where an equilibrium lays. For a given reaction, if $K$ is larger than 1 than the equilibrium is towards the right. If $K$ is less than one then the equilibrium is towards the left.
   For a simple equilibrium such as acid dissociation, the forward reaction's equilibrium constant is

$$K = \frac{[\ce{H+}][\ce{A-}]}{[\ce{HA}]}$$
so you can compare the strengths of acids by comparing the equilibrium constant. Since the $K$ of nitric acid is larger than the $K$ of hydrofluoric acid we know that it is a stronger acid.

Hope this helps!