The primary purpose of serial dilution is to determine the concentration of an unknown substance by diluting it in a series of steps, and then counting the number of visible units or colonies in the diluted samples. By analyzing the counts obtained from the dilutions, one can calculate and estimate the original concentration of the unknown substance. Additionally, carrying out a serial dilution assists in adjusting the concentration of a solution to achieve more manageable levels. The process involves gradually decreasing the concentration of the microorganism.