Using your bathrooms vent fan can do a few things for you: it can keep humidity levels down throughout your home, keep condensation down on your windows in the wintertime, and give your A/C a break. When your A/C has the added job of removing the heat and moisture added by your shower, it takes a lot more energy than if you were to just turn on your bathrooms vent fan for 20 minutes.
Doing this will save 42 lbs of CO2 per year! That’s preventing the same annual impact as powering a hot tub for 4 hours, charging 6813 batteries, or charging a cell phone 7630 times. (JouleBug)