If you only want a little general knowledge about a subject, yes books like that can be way overkill. But if you are doing research into something, perhaps a specific event with a narrow timeframe, then you need a lot more detail. Also, diaries like Pepys' is very useful for someone studying that time period - knowing what he had for breakfast (and perhaps what his acquaintances had for breakfast) tells us more about people living then. Helpful for both historians and fiction writers. Eventually that knowledge will filter down to you through the Wikipedia page or a historical novel in a more generalised form. It all depends on what you need to know the stuff for
