Much like the way that pH is written using the logarithm function to convert the actual [H^+] of a solution into a simpler value, so the K_a of an acid can be converted into a pK_a value.
Once you have the K_a value, the applications that can be made from K_a or pK_a (whichever you prefer to use), include the ability to determine the [H^+] in a solution of that acid, regardless of the concentration of the solution, or whether it is buffered for example.
K_a is an equilibrium constant, and as such is independent of the concentration of a solution. This makes it the best value for establishing the strength of that acid relative to others.
