Mendelsohn explains the famous Herodotus’ digressions as means for the history of the Persian Wars to be incorporated into a vision of a cosmic history or of the nature of cosmos (world/existence). Mendelsohn seems quite right to me; I’d like to add only that Herodotus didn’t give birth to History out of nothing. A cosmic vision exists already from the time of Homer and then in Hesiod (in the latter even a History of the Cosmos).
Herodotus’ achievement was not something new, but on the contrary, it was the fact that he managed to found historical analysis inside the philosophical / poetical demand of a greater scale of thinking. This achievement was then followed by Thucydides. Thucydides may sound more ‘clinical’, but he speaks about the nature of man, he does not describe with the greatest possible accuracy historical events, he refers essentially to the future, and in his analysis he searches for natural laws governing this history, making of his material exemplary/characteristic/ideal appearances of the laws of a history which is formed according to the nature of human beings.
There is a relative decrease in the importance of the metaphysical element as we go from Homer, to Hesiod, to Herodotus, to Thucydides, which corresponds to the increase of the importance of the historical element. Homer already new how tragic history is, and the later generations were going to explore this tragical sense, never doing history just for an exact description of facts. Their aim was to explore the tragical nature of history, an aim which included a metaphysical stand – even when that was not apparent.
Daniel Mendelsohn: What was Herodotus trying to tell us? – Excerpts, selected by Ellopos (full text here).