What is the use of Horner's Method?
Yes, exactly as you said. This is used to decrease the number of multiplications, so it is more efficient than computing it the normal way.

Example:  


  You have the polynomial $ax^3 + bx^2 + cx + d$.
  Computing directly you have six multiplications: (a * x * x * x, b * x * x, c * x). With Horner $((ax + b)x + c)x + d$ you have three multiplications: (a * x, first parenthesis * x, second parenthesis * x)


It is important to remember that in floating point calculations results from these two methods (direct and Horner) will differ.