I like the food because of the filtering function it can provide.
Veg*nism, although always having a certain connection to food by definition, has many different aspects. Having a tag like food helps to search for either only food related or no food related questions. I feel it is important to have tags with different levels of granurality (and general applicability) as they can provide different sorts of view on the topic.
Imagine having tag meat, or not having tag meat at all and just typing
meat into the search box. You would likely get questions like
Am I still vegetarian if I occasionally eat meat? and questions like
Is there any vegan food that is nutritionally equivalent to meat?
These questions are miles apart when we consider both their subject and the possible audience. One cares about the ethics, the other about diet. They are both correctly shown when searching for
meat. But it is unlikely that someone will want to see both of them pop-up in one search, at the same time.
Having a tag such as food helps the user to define his interest or disinterest in a certain aspect of veg*nism when performing his search. It helps to provide a way of effectively finding relevant information - the very purpose of tags.
It is true that a lot of questions can (and will) be tagged by this tag. But for me, that is the point. Because its absence will then be as meaningful as its presence. Because it will be a simple big-picture filter that can be used to drastically (precisely because of its generality) improve the search results.
If it was the only tag used for searching for something then yes, it would not be very helpful. It is by the combination with other tags that it really shines.
My answer presupposes a site that is somewhat bigger than Vegetarianism SE currently is. Meat tag, for example, currently shows 2 questions..