When most people think about air pollution, they think about smog, pollen, and other outdoor air quality problems.
The next thing that probably comes to mind is indoor air quality, which usually becomes a bigger problem during especially hot or cold weather, when dust and other allergens are allowed to collect inside homes and businesses that are sealed up against the elements.
The fact is that indoor air pollution is also a problem inside cars, so the idea of car air purifiers actually has a lot of merit. All of the same pollutants and allergens you find outside are present inside your car as well, in addition to chemicals and particulates that come from the car itself.
For instance, an analysis performed by the National Institute of Health found that particulate matter from brakes, and aromatic hydrocarbons that come from various interior components, can cause health problems. One solution is to simply roll down a window, but that just allows all of the pollution from outside the car to get inside.
Another air quality issue that a lot of people have to deal with involves lingering smells from tobacco and other sources. Purifiers and ionizers usually won't help with this type of problem, but you may have luck with adsorbents or ozonators.