The only reason to have file extensions would be if you're serving actual files, and the server determines the type of file based on the file name. For the Ranch URL you cite, there is no actual file -- the page is generated dynamically by a servlet. It's not that there's a file with no extension, it's that there's no filename involved at all.
They are semantic. Jeanne pointed out the main advantage.
The advantage the extension had (sic past tense), besides when serving static files: Broken servers and clients (i.e. browsers) treated a document by extension, not by content type. So in earlier times some browsers would offer you to save a file instead of presenting it because the browser didn't know what to do with it without extension. Oh, and there where some server that determined the content type by extension and set it "automagically" so you didn't have to care for that. One of the situations where it would have been better to let the system fail so the cause gets fixed.
Broken clients, broken servers. Long time gone.
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