This method is very simple and easy to understandable and also it does not require much of calculations.
The method is used only when the trend in linear or almost linear. For non-linear trend this method is not applicable. It is used on the calculation of average and the average is affected by extreme values. Thus if there is some very large value or very small value in the time series, that extreme value should either be omitted or this method should not be applied. We can also write the equation of the trend line.