Should I get a cool mist humidifier?

I'm not sure if I should buy a humidifier or not because I'm not exactly sure what they are used for. I read in what to expect the first year that cool mist humidifiers are best but it doesn't explain what it is used for. What are the pros and cons of a humidifier and what is it used for? Also I am living in south Florida and my baby will be born in November (I'm not sure if this matters) can anyone please give me advice ?  
Btw I'm posting this as anonymous because I'm a little embarrassed that I don't know what this is used.