Importance of duct in bathroom exhaust fan?

The moisture needs to be extracted outside so it doesn't cause damp and rot things

In cold climates the space above the insulation is freezing, the added moisture will form icicles on the tips of all the roofing nails and then eventually melt and drip onto the insulation.

In theory, if you have enough ventilation inside the attic, you don't really need a roof vent for the fan exhaust, but your local mechanical or fire code may require it anyway.

Ideally, the vent should go to the outside. However, if you are in the hot south like Texas or Arizona, it's not that big of a deal if you vent into the center of the attic. This is assuming you have good attic ventilation and you do not blow the warm-moist air directly on any wood.

http://www.atticfoil.com/

http://www.radiantbarrierguru.com/

Having a lot of humidity in the attic is not a good thing.

putting to much moisture in the attic is not a good thing

thats the lazy way ..and wrong ..the exhauted air is moisture laden it should always be vented to outside