To simulate the resistance of the potentiometers to ground. The volume pot turned to 10 is the equivalent of a 250k or 500k resistor connected to ground connected to the pickup output. Likewise the tone pot turned to 10 is the equivalent of a 250k or 500k resistor in series with a capacitor (normally between 0.22uF and 0.47uF) connected to the pickup output and wired to ground.

So you can simulate the effect on the sound of changing the pots and capacitors with all the controls turned up using very cheap components. You can also do it fairly quickly so you don't forget what the previous combination sounded like (though making a recording can help as well).

The block's only there so you can change the resistors and caps with a screwdriver and not a soldering iron.