RF in/out - I assume the point of it is so that you'd only need one lead going from the STB to the VCR/TV, carrying both the STB's signals & the analogue aerial signals. Of course, I would've thought it's less use if your TV & VCR have SCARTs, as the aerial could just go straight to the VCR/TV, while the STB would be SCARTed up to the VCR/TV.
Ethernet - nope, AFAIK it's solely for the in-built cable modem, for if you receive broadband via the STB instead of a standalone modem.