Explain why a buffer solution is formed when excess weak acid is added to a strong base

Because the weak acid is in excess, all of the strong base will react to form water and a salt of the acid. Since there is still some of the acid in the reaction mixture at the end point, an acidic buffer is formed as the equilibrium between the acid and its disassociated ions is still present, which is what allows a buffer to resist changes in pH. However, the addition of the extra salt formed in the reaction with the base is what makes the solution a buffer as it increases the volume of acid or alkali that can be added to the solution before the buffering effect is overcome.