The idea is that there will be current between the chassis of the preamp and the turntable. This is usually due to capacitance between the power line (connects to the turntable motor and wiring, and to the preamp power transformer primary) and the chassis. You want to keep this current out of the signal grounds from the cartridge, so you connect the chassis to each other with a heavier wire, and you run the signal grounds to a single point, usually right at the input of the preamp.
If this current ran through the "ground" of the interconnect, its non-zero resistance would create a hum voltage that the preamp would see as part of the signal - it can't tell the difference. We call this a "ground loop" but it's not usually due to magnetic fields as the textbooks say, it's usually the capacitive coupling to the power line.
