I own more than half of these old games, including Waterloo, Afrika Korps, Third Reich, and Alesia. I find this tag very useful to highlight games and questions that I am interested in following, rather than having to tag all the games individually.
There is an entire generation of Grognards that grew up with these games, on which we cut our competitive teeth, for whom this tag makes eminent sense. There is even an annual tournament hosted by the Boardgame Players Association, the World Boardgaming Championships, which is dedicated (primarily) to these old classic games. Waterloo and Afrika Korps in particular are regarded as classic evenly balanced contests.
Also, as many of the classic wargame titles can refer to one of many different games, of different styles, age, and complexity, the tag assists in identifying which of many possible games a questioner is inquiring about.
Further, I propose that anything that helps to leaven the current MTG-emphasis of this site is a good thing. Nothing against MTG - I enjoy playing it with my son - but the site is over-weighted in MTG questions for a general-interest member.
Finally, for any gamer older than about 50, the term Avalon Hill can only have one meaning - the classic wargames produced by that iconic company between the release of Tactics II in 1954 and it's purchase by Hasbro in 1971