AirDoctor has a built-in ionizer, but the feature must be turned on as it is not on by default. The ionizer helps with revitalizing the air by infusing it with negative ions. It essentially clumps contaminants together so that they are larger and therefore, easier to capture. Some people enjoy creating this effect, but it is a completely optional feature. AirDoctor will work perfectly to purify your air, even if you never choose to use the ionizer option. AirDoctor does not depend on the ionizer to achieve its high-level of air purification.
Here is a video on how the ionizer works and when to use it: