html tags always have a class?

When I edit books that I buy or download from Project Gutenberg they always have classes on almost every html tag. For example, every p tag has the class pindent or some such.

Then the CSS will have something like ".pindent { text-indent ...}". When I'm fixing a book, in the CSS I'll just have a plain p with the necessary formatting; "p { text-indent: ...}"

Is there some reason, for compatibility, safety, etc. for them doing that?