Humidifiers are mostly used in areas with cold winters where the forced air heat from natural gas furnaces makes the air extremely dry. My sister lives in such a place - and needs a humidifier to prevent asthma attacks.
Humidifiers can harbor a lot of bacteria - maintenance is important. My sister uses a UV (ultra violet) unit to kill bacteria in the HVAC system of her home.
Luckily we can open our windows here to allow for normal humidity.
There are also desert areas that use 'swamp coolers' which are rooftop evaporation coolers that are used in the summer. Evaporation of these units to prevent disease is also important.