Hi, history is more interdisciplinary than the question seems to imply - but it's also a big question.

Cultural history may involve study of fictional literature; for example, a student could examine how Shakespeare's portrayal of the Plantagenets in his history plays related to propaganda reinforcing Tudor / Elizabethan legitimacy. There, literature is a central part of historical study.

In other instances, it may be a way of locating supplementary details not recorded or with minor coverage elsewhere, particularly in the area of social history, customs and everyday life. It is certainly more useful for older history, particularly medieval and early modern, where there is not such a glut of evidence as there is in, for example, the second half of the twentieth century, but in most cases it isn't a binary either/or. And it must always be borne in mind that literature is fictional or mythical, and that there may not be ways to check whether a social habit, for example, was widespread, or fictional. A historian might cite an example from a well-known novel to liven up a book, if it is a good and relevant example.

But it would not be good practice at all to cite, say, a historical novel about the Second World War when writing a factually-based assessment of military planning: there are countless better secondary sources, never mind primary ones, and there may be artistic licence used.