I think it is just a design choice. My VPI has a junction box with the L/R RCA connections and a ground connector. My TTW GEM does not. The phono cable should have one ground which can be connected either to the turntable ground or the phono pre. If you have hum, then you can try to connect the turntable ground via a separate ground wire to the phono pre. Ground loops and/or hum are problems that can be infuriatingly difficult to fix. With my VPI, I had to use one specific pair of phono cables, as I got a hum with others. For the TTW, I found that no matter what I tried, I couldn't get rid of a hum when using one cartridge. Other cartridges play fine with it.