But that's based on your own system, which may be non-obvious to other developers. The rules of valid HTML dictate the situation in which you will encounter a given ID, so this communicates something to every developer who sees IDs used in a given selector. To me that's tremendously valuable, and far more useful than trying to graft additional meanings onto parts of a single classname.
Whether that system is BEM, OOCSS, SMACSS or some other syntax, this kind of naming convention isn't a native part of CSS.