Dirty brushes become more abrasive and dry as they become more caked with product and debris from your face. In turn, this irritates your skin. Cleaning your brushes regularly keeps them soft enough to not cause damage to your face. Dirty makeup brushes are also a breeding ground for bacteria.
You could get a staph infection.
Dirty brushes will make your makeup will look worse.
You risk getting more wrinkles.
You may attract bugs.
Washing your brushes keeps brush hairs in optimal condition. Monthly cleansing is recommended to ensure the highest level of hygiene. Your brushes will be happy and in return last longer.