Product review (4 stars): Necessary accessory
These help reduce the mineral buildup on the heating element in the humidifier.  We've tried a lot of humidifiers for our house and our piano room, and the Optimus have worked the best.  You have to clean the heating element regularly, however.  The absorption sleeves help, but the glue holding the velcro fasteners seems to decay with heat, and so the sleeves wear out faster than you might expect.  We've also used filtered water to reduce the mineral buildup and the cleanup problems.