What does a diffuser do?
I usually let my hair air dry, and it turns out okay-ish. I'm thinking about getting one but I don't want my hair to look poofy.
Diffsers diffuse the air flow, meaning they increase the area that the air is coming from, softening the air flow and allowing you to dry curly hair without causing too much frizz. The narrow nozzle on a hair dryer causes and strong stream of air to push and blow around the hair, which becomes unable to control without a brush.
A lot of curlies diffuse their hair til it's about 75% dry then let it air dry the rest of the way. I really like this method for my curls.