Explanation Of Fans And Carbon Filters

I know there was to be an intake bringing in air or an exhaust moving out warm air. Why do people put carbon filters on the intake, it doesn't seem to make sense. Also, is it better to have the carbon filter on the inside of a grow box or the outside of the box on the exhaust fan?

It's more efficient to pull through a filter than it is to push, that's why they come first and the fans draw air through it, clean it, and exhaust it.