The hiss and hum that you hear is very low power interference from radio waves that are floating around from local lighting, dimmer switches, the wiring in the walls, in fact anything that consumes electricity will leak some radio frequencies. Guitar pickups produce a relatively low current/output which is transmitted through the guitars internal wiring and out through your cable to the amp where it is amplified greatly. Any un-shielded hot wires will pick up radio interference and transmit the signal along with your guitar signal to the amp where it is amplified and so when you are not touching the strings and grounding the guitar all you hear is the background noise.
Of course this is annoying in a live situation and becomes a real problem when recording.
What this and the other kits that I list do is to enclose this wiring in a conductive box which is connected to earth so that the interference does not get to your wiring in the first place.
STRATOCASTERS are probably the worst for this because of the amount of wiring spread out over the guitar, the 3 single coil pickups and almost complete lack of shielding, Leo Fender being a radio engineer knew a bit about interference and originally built his guitars with a full die cut aluminium shield which has over the years through cost cutting, shrunk to a tiny patch if thin aluminium foil, which is pretty useless. You can render your Strat almost silent by shielding the pickup and control cavity with shielding paint and the pick guard with either copper or my full Die Cut Aluminium Shield.
TELECASTERS are actually better because the neck pickup has a metal cover and the bridge pickup is attached to an earthed bridge and the controls are on an earthed metal plate but as you know interference still gets through so shielding the control cavity will provide an improvement.
HUMBUCKER equipped guitars, the clue is in the name, have pickups that don't produce much hum but that still leaves the wiring in the control cavity and it really depends on how well your guitar is wired. If you have un-shielded control wires then they can pick up interference, If your guitar has all braided wire then you should be pretty good but if hum still occurs you can benefit from shielding the cavity with paint and the cover with copper foil.