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.
