A lot of self-hosted sites are standalone and that is the default configuration, so the logo should, in those cases, always bring you to “home” of the community.
Adding Discourse to another “website” (it’s a separate app mind you!) is of course a reasonable think to do but it’s far from the only and base use case.
NB you can now display entire custom HTML pages within Discourse via plugins so Discourse can become the “entire” website, e.g. Landing Pages Plugin 🛩 - #95 by merefield