Can You Help Me Understand My Air Purifier?

I recently bought an air purifier. it has a switch that, when set to that, sends negative ions into the air. I read the instruction booklet and it said that the negative ions attach to dust and make it easier to purify. It said they were natural and harmless. Great, i thought.
It has another setting that turns off the negative ions but still leaves it running. Why would you want to? If they are so helpful? Should i be only using the negative ions for part of the day? Is there some reason to have it working with out this feature?