Why do you need to change the air filter in your car?

Why do you need to pick your nose? To be able to breath better? What if you could use a nose 'filter'.. one that collects all the boogers where you then pull out the filter and clean and/or replace it so that it won't clog up and prevent you from breathing freely. Vehicle air filters work that way as well, it doesn't collect boogers but instead minute particle which would otherwise (over time) affect the performance of the engine.. From time to time it's necessary to clean or replace the filter else the particles will clog up and hinder circulation.

The same reason (hopefully) that you take a shower. Because it gets dirty.

The filter in your car is a mechanical devise that eliminates the dust (that is in the air) from entering the engine and doing harm to it.  At a certain stage the filter is "full" with dust particles and either don't let clean air to the engine or blockes any air to it.   That is the time to change the filter (or clean it properly).  The question is: When to do it ?  The more you go on dust roads and the more your car is exposed to dust the more ofter (frequent) you should do it.