In an evacuation event, the aircraft would be at sea level and the interior would not be pressurized relative to outside air. You only need to depressurize if you're going to parachute out, which... is not an option in commercial aviation.
Pressurization is relative, aircraft can end up pressurized on the ground. Next time you fly, note the warning beacons in the windows of all the doors. They indicate the cabin is pressurized alerting rescue crews that attempting to open said door will either A: not work or B: kill them, as has happened before with a G150 [1]
Also, there are other situations where you may be inclined to disable bleed air, such as in a fume event where an engine is releasing turbine oil into the bleed air stream due to a mechanical failure or fire.
Aircraft are pressurized to 8k feet above sea level. Denver is only 5k. La Paz, specifically, sure. It’s a lot less broadly useful than your original language made it sound.
