Meta tags have basically two usages: the main usage is enhancing information. The second though most used function is improving visibility in search engines. Both are evidently (implied in the question) suppressed by other means.
Quoting the first note about meta tags on w3.org:
Note. The W3C Resource Description Framework (see [RDF10]) became a W3C Recommendation in February 1999. RDF allows authors to specify machine-readable metadata about HTML documents and other network-accessible resources.
That already says a lot about using meta tags for its main purpose. Also there's no reason to add hidden relevant information on the HTML. It's better to just add them to a sitemap, faq, info, about, anywhere within the website structure, visible both for robots and people.
This website analyzes the SO page design in deep, and the guy also gives a good analysis on why not adding meta tags for search engine, and using Robots.txt instead. The SO folks do use robots.txt and also do some optimization for search engines. So, no good reason to insist on meta tags for SEO.
As for few more specific usages:
That's not to say meta tags are utterly and completely useless, but I believe they're used in Stack Exchange due to a mere lack of standard within the SO team and there's no real need for them.