What is sound deadening?
Sound deadening is a way of making a car interior quiet, reduce rattles, road noise and vibrations. This will increase the ride comfort by reducing noise, heat and improve the stereo sound quality.
Imagine your car, now image that all the rattles and the road noise are gone and you can whisper to your passenger instead of raising your voice. This can be achieved with sound deadening. There are multiple types of sound deadening materials, but they try to achieve two things: block noise and heat.
A car without rattles or road noise will feel more comfortable, it's quite a sensation when you go thru a pothole and you don't hear any rattles or the only noise you hear is the suspension working with a muffled sound.
At highway speeds the wind noise can cause fatigue, you can prevent this with sound deadening. A simple 3 decibel reduction can make your car interior sound twice as quiet as before.
Why don't cars come deadened from the factory?
Almost all cars do have some sort of deadening material applied, but most of the time is quite inadequate because deadening adds cost and weight. If you have a city car there is probably almost no sound deadening to keep the car light and cheaper.
Luxury cars will have a lot of deadening material to give that luxury ride feeling. Rolls-Royce for example uses a butyl based material very similar to Dynamat Extreme.
Some manufacturers will even add laminated side windows (called double-glazed by some manufacturers) to further reduce the noise.
Luxury is nothing without cool and quiet. You can have leather, wood, but if you have a noisy and hot car what difference does it make? It will be uncomfortable.
What materials can I use?
It depends on what you are trying to achieve and what kind of noise source you want to block. Butyl is usually applied to stop panel rattling, while MLV and dense foams will block air borne noise.
Dynamat Extreme blocks noise and some heat while Dynaliner blocks heat and some noise.
You can browse the steps I have done on my cars and see the products that were used or if you want something more technical take a look at Sound Deadener Showdown to know the type of materials to use or Guia da insonorização automóvel (in portuguese) for a quick guide that I made for someone who wants to deaden their car.