The overview file allows a quick access to the newsgroup overview. There are feeders for statistics gathering, but I haven't looked at them. Files used by INN Spool spool actually stores articles. It is not recommended anyway. It is a kind of log file system, but much simpler.
Articles stay on the disk until it's overwritten by another article.
Different newsgroups can be configured stored on different types of spools. The history file lists each article that server has. The plain history is used by expire process. These files are updated only by innd and are deleted only by expire . It is usually kept entirely in innd's memory and flushed every now and then.