Product review (3 stars): It's a wick, and it rots.
These "filters" wick up the water from the tray in which the water tanks sit and into which the tanks release the water.  The unit's fan sucks air through these wick-filters and blows the moistened air out the top. So even if you clean the humidifier tanks and tray regularly and well, these "filters" are necessary for the humidifier to function properly. If they are not changed every couple of months they will mould and rot, and you will freak out.