I'm debating buying one for myself so I can judge whether air pollution is effecting my asthma. You can find AQI for a lot of cities but it appears the calculation often isn't done with local devices. Satellite imaging?
CO2 is important to track as high concentrations can impact sleep quality and critical thinking ability.
VOCs is important to track as they are associated with asthma and allergic reactions.
CO2 can only be removed by opening a window.
Most VOCs come from household chemicals and are only removed by using lots and lots of charcoal filters as the HEPA filters in household air purifiers aren't able to capture it in significant amounts.