We have modified the earlier 'package' custom function because most of the other parameters like 'house rent allowance', 'travel allowance', 'provident fund' and 'medical allowance'. Now the calculations are easier and quicker. The disadvantage may be that if you don't know how the 'salary' multiplications have been used. For example, if the remarks were not there then how would you know that 'salary*0.6' belongs to 'house rent allowance.
Although Excel has hundreds of inbuilt functions there will definitely come a time when you need to create custom functions. For example, there is no inbuilt function to calculate the area of a cylinder or circle. You may wish to create a custom function to calculate the average mileage that your car gives based on distance travelled and gasoline used. Thus custom functions will always play an important role in many areas of work.