It is done to eliminate/reduce 60 cycle (50 cycle in your case) hum when using single coil PUPs
I used self adhesive copper foil (with conductive adhesive) for doing this. You can find it at various guitar parts companies such as Stewart-MacDonald. There is also a shielding paint available. Some people use heavy duty aluminum foil.
I use adhesive aluminum tape, the type that is commonly used for HVAC work. You can purchase it at Lowe's Hardware or Home Depot relatively cheap. One roll will do a number of Strats. You can also use it to shield the back of a pickguard.
When I got my '87 LP Custom, the previous owner had the cavity sprayed with a sheilding paint that I believe has nickle in it. Check Stew-Mac and see. The foil is good, but can be a little hard to work with.
No, you only need the cap for a tone knob. Well, there are other instances, like a treble bleed circuit, but for a basic volume knob it's unneccessary.