It is generally important to be well versed in any thematic field that one try and participate in, but not only because of the old saying "History is written by the victors" even though the fact that it isn't entirely certain who actually said that first, lend some considerable credence to it's truth.

When writing about history, so much can be wrought and changed. Diplomatic letters can be forged, writers can be wrong, cited sources authors had only seen once, and perhaps a decade before writing isn't an exact science to put it mildly. Scribes can be wrong. Authors can lie or simply trying to beautify the subject. The written source may be hearsay. It might be oral history warped by generations etc. It isn't alway intentionally wrong, but most sources taken alone is pure bullocks. That is why the historical discipline have had a long and indeed very prosperous relationship with various other sciences, disciplines etc. Philology is a great example, ex. the early works dismantling the [Donation of Constantine] (http://en.wikipedia.org/wiki/Donation_of_Constantine) a brilliant feat by Lorenzo Valla, or [Isaac Casaubon] (http://en.wikipedia.org/wiki/Isaac_Casaubon) that dated the Hermetic Corpus to well after the birth of christ, when scholars around Europe had thought it to be a remnant of Ancient Egypt. Especially today archaeology, biology even physics are shedding new light on our present sources. But something as simple as the knowledge of heraldry might be a way to date or validify a source. Regarding the thematic field scholars partake in, they often have a very sound overview of what is the general trend in modern academia aswell as an idea of it's historiography, which is the discipline of how history has been told.