People can read you like a book easier because it exaggerates your facial expressions esp if it's to do with the eye area. So if you're surprised the raised eyebrow is highlighted and if you're disgusted or a little bit angry that really shows. I mean if you had sparse eyebrows it is still readily apparent but the thick eyebrows would highlight the expression and give it extra oomph. It's like putting text in bold rather than ultra light which would emphasise the reading or in this case the facial expression reading