Too much moisture indeed sets your home up for a mold-breeding playground, but not enough moisture can have its own adverse side effects.
Humidifiers register the amount of moisture in the air. By attaching to your HVAC system, whole-house humidifiers bring water vapor to the ducts and distribute that moisture throughout your house. You set the moisture level from a thermostat and forget about it.
When you start to see the signs of dry air, it is time to consider a whole-house humidifier. Winter is already dreary and cold; why make your chances of illness and uncomfortable skin an added disadvantage.