Overview
Sound deadening is panel control, not magic silence.
Butyl mat helps stop sheet metal from ringing like a drum. That is useful in van doors, quarter panels, wheel wells, floors, and other broad panels that vibrate.
You do not need to wallpaper every inch. Strategic coverage on clean metal, rolled down properly, usually does more than sloppy full coverage.
It is not thermal insulation and it is not a full acoustic build by itself. It is one layer in making a vehicle feel less tinny.
Use it where panels ring. Stop when the panel is dead.