DNS is everywhere. It offers massive caching potential. It is well-understood and there are multiple, interoperating, implementations. It is a data diode, which is to say read-only, which can simplify some risk assessments related to attack surface.

The README for this project may explain some of the general benefits (my project): https://github.com/m3047/rkvdns

HTTP is everywhere. I think SOAP sucks, but it's everywhere and it works well enough. (For that matter XHR sucks, but even I use it occasionally.)

(This is not the first time DNS Toys has shown up on Hacker News, there's a link in another comment.)

Often passed through firewalls or effectively proxied through resolvers

For fun.

Because it is fun.

dns clients come preinstalled on virtually any os