How and why do we replace activities of reactants and products by concentrations in equilibrium law equations?

Question

How and why do we replace activities of reactants and products by concentrations in equilibrium law equations?

Answer 1

Because at most (but certainly not all) concentrations and activity and concentration will be numerically equal.

As solutions become more concentrated then the values for concentration and activity begin to differ more and activity should be used in calculations rather than concentration.

Answer 2

How we do it is we just do it.  Why we do it is to make everything easier for calculations.  We make certain assumptions that simplify the overall procedure.  Thus we use concentrations instead of activities or activity coefficients.  

Answer 3

Theory tell you that you have to work with activities but they are difficult to determine. Because they are proportional to concentration one simply insert concentrations instead of activities.