Obscure arguments in the TrainingArguments class

The TrainingArguments class has arguments do_train, do_eval, do_predict that “are not directly used by the Trainer class”.
They are allegedly intended to be used by my training scripts. But how? If I write a training script to train a model it will train the model. Why do I need an extra argument for that?
The examples link does not point to any specific example with explanation.