Many people use tags to filter out questions they view. (For example, using favorite tags, ignored tags, filters, by regularly visiting some tags or following them via e-mail notifications, by viewing only questions in selected tags, etc.)
One of the effects is that some questions might go unnoticed because they are only tagged by some obscure tag and they are not using any of large tags with many followers. In other words, correct tagging can influence how many people actually see the question.
Do you think that it would be useful to have a set of tags such that every post on math.SE should contain at least one of these tags? If yes, what would be a reasonable candidates for top-level tags?
Apart from the possible problems with low visibility of questions using niche tags, top level tags might also help with better organization of the questions on the site and with searching. (For example, if all questions on lp-spaces are also tagged functional-analysis and you are looking for a question which definitely belongs in (functional-analysis) but it might be also tagged (lp-spaces), you would only have to look in the top level tag.)
I do not think of this as some formal policy, which would be enforced by software. I simply suggest, that if we have a list of such tags, users who retag questions could add one of the top-level tags if they are missing.
Probably some of the tags in the among the most popular tags might be reasonable candidates for top-level tags.
1I use the name top level tags in the same meaning as on MO. I do not know whether this phrase has some special meaning on some other sites in SE network.