From Ed Del Grande's blog:
"With all those plumbing fixtures packed into a small space, most bathrooms will inevitably be very moist areas. To my dismay, many homeowners and their family members never use the exhaust fan for their bathroom unless it's wired directly in with the main bathroom light switch. I bet some of you believe that you are using your exhaust fans properly, and think the fan is just meant for eliminating bathroom odors when company is over. While your bathroom fan does help to control normal odors, the real reason for an exhaust fan in your bathroom is to control the moisture."
For more: Ed Del Grande: Ask Ed -- Why Is a Bathroom Fan So Important? (HGTVPro.com)