It depends greatly on your climate. I didn't need one in the midwest, where air quality is pretty good and there's a lot of humidity in the air, but when I moved westward I definitely noticed the difference in air quality (my asthmatic symptoms came back which I haven't had since I was a child).

Especially in autumn when the wildfires start up - even if it's many miles away there is a very noticeable degradation in air quality.