Titanic “incomplete” data samples

Could not search threw the forum(i’am kicked to the google from the search bar). Can somebody explain, why all kernels advice to complete the Titanic people with mdeians and mods? I mean, we can afford getting rid of incomplete data, so the classification will be more clean. Yes, we CAN complete the test data with medians and mods, but isn’t “a bear’s favor” – to complete the data in train?

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