You dont always cross-validate! Yes sometimes after you do the train-test split u will use something like GridCv in sklearn to cross validate. But think of having to do 5-fold cross validation for a large NN model taking 10 days to train..you now just spent 50 days! So there you take the remaining training set after the test was left out (if u left a test out) and split into a smaller training set and a validation set.