Question of eval_mask of airquality dataset

[RapperYao]

As mentioned in the article, only the data that is valid and invalid at the same time in the next month is used as evaluation data. I do not understand this setting. Why can't all the data except the missing value be used as evaluation data? Doesn't this limit the model from learning a broader representation? In addition, a whiten_prob is mentioned in the code, I would like to know if this whiten operation is only carried out on eval_mask