We usually employ this method during lab experiment. Reaction of solutions of HCl and NaOH produces heat. In this case, we are to measure the heat of reaction by getting the change in heat of the solutions once mixed.
Preparing solutions of NaOH and HCl have varying temperatures and the results may vary. By letting the two solutions sit overnight, the temperatures of the two containers will equilibrate to a certain room temperature. Now, the temperature will not interfere with the results since they have the same temperature when it will be mixed.
The chemical reaction in mixing the two solutions with the same concentration can be written as:
HCl (aq) + NaOH (aq) -> NaCl (aq) + H2O (l)