Journalism is important to historians because it can give us a window on to the past. It can tell us what people were thinking and saying at various important times in the past. It is for this reason that journalism is sometimes called the "first draft of history."
If you look in any history textbook, you will see examples of how journalism is important to history. The most obvious example will come in the historical political cartoons that you will see in the texts. These cartoons are used to show you what the people in a given historical time period worried about (what issues were important to them) and how some people, at least, thought about those issues. The same thing can be done with the text of old newspapers and magazines, though this is not so clearly visible in your textbooks.
Because historical journalism gives us a snapshot of what was seen as important at any given time in history (at least since journalism existed) it gives us a great way to know what those times were like. This is why journalism is so important from a historical perspective.