The high-low method has two advantages, First it is objective.
That is, any two people using the high-low method on a particular data set will arrive at the same answer.
Second, it is simple to calculate. The high-low method allows a manager to get a quick fix on a cost relationship using only two data points.
For example, a manager may have only two years of data.
Sometimes, this will be enough to get a crude approximation of the cost relationship