The more I see these agenda/to-do lists/productivity apps, the more I feel I am missing something. What is wrong with just using... your agenda? And if stuff n

The more I see these agenda/to-do lists/productivity apps, the more I feel I am missing something. What is wrong with just using... your agenda? And if stuff needs to be broken down into little tasks.... out them in the tasks app. Why buy a complicated piece of software, if the aforementioned apps are already on your phone, for free. Also, bonus: these apps synchronize between iPhone and Mac seamlessly.