Having contributed to the answer regarding spelling, let me try to answer this question for real, though I'm not sure how to provide sources.

Diaries are, generally speaking, very important to historians. Typically we have a high-level view of the actual events in a period, but a collection of diaries or other personal papers fills in a lot of gaps, and not just in terms of knowing what people thought of what was going on. Most historians don't just study the prominent events of history, but instead spread out into questions regarding some aspect of the day-to-day life experienced by people in that time.

If you were to keep a detailed diary now and then checked back in 300 years, I think you'd find that your diary wasn't just important in helping historians figure out what you (and by extension other people of your social and economic class in your polity) think about what's going on around you, but that it also helped a huge amount in determining what you actually do on a day-to-day basis, and the importance of those things. Diaries contain details that can even help historians identify the occurrence of certain kinds of unusual phenomena, like weird astrological occurrences.

Now, diaries aren't universally useful to historians. As with many things, how useful any given diary is, or even how useful diaries in general are, depends on what you're studying. If you're studying agrarian village life in a period of low-literacy during, say, the early Middle Ages in France, the usefulness of journals and diaries isn't going to be as useful to you. As another example, an economic historian is more likely to find things like deeds of sale and church records of land ownership to be useful.

Even then, diaries are still supremely useful for contextualizing the more concrete things we learn from sources that provide hard data. Going back to our example of an economic historian, if I find evidence for massive economic slowdown in the sources that provide me numerical data, a good place to start looking for reasons why there was an economic slowdown is a collection of diaries from the period. Even if nobody says outright "business is bad because a harsh winter ruined the crops", the researcher can still reasonably draw the conclusion that everybody mentioning what a bad winter it's been at the same time we've got evidence of economic downturn means something significant.

Source: my education as a historian and my employment as a rare books librarian. I can find some good books on historiography, especially the study of cultural records, if you or anyone else is interested.

A diary would be considered a primary source. Meaning that it is a document that comes directly from the time being researched. As a historian the use of primary sources is essential to building an argument that can cite these primary sources as direct examples that can support or refute a thesis.

In my research I've used a diary from young lady that lived in my town while it was occupied by union soldiers during the American Civil War. This gave me her location, her opinions, the road she walked to get to town, the location of soldier outposts, what she went to town to get, what supplies were available or in shortage, and much more.

A diary can be extremely useful depending on what information you are looking for and what your thesis is about.