Water droplets all over the beams and insulation
When the humidity in the crawl space gets too high, it condenses and forms up like rain. The smells are musty and the air is most likely filled with mold spores.
Wide spread damage in the crawl space
You can see that the problems aren't located in one spot. When this is happening under your home it will effect things like allergies, heating and cooling costs, and your overall comfort.
Hanging insulation in the crawl space
When the insulation hangs down, or even falls out completely, it is no longer doing what you want it to do. At this point it is ruined. The air ducts are also going to pull in the nasty smelling and wet air. All the air will be spread around the house.
Fungus and mold growth
The wood damage is pretty clear in this picture. When all the fungus, insects, and mold attack your support beams, the floor weakens and causes foundation problems.