A diffuser doesn't really humidify the air like a humidifier would.
A diffuser just puts out a mist of the essential oils and releases those essential oils into the air through the mist.
A humidifier pushes out moisture and regulates that moisture in the air.
When you need to add humidity to the air you should use a humidifier made for the purpose but it's also beneficial to use both a diffuser and humidifier at the same time.