This method is very simple and easy to understand, and also it does not require many calculations.
The method is used only when the trend is linear or almost linear. For non-linear trends this method is not applicable. It is used for the calculation of averages, and averages are 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.