Face Shields cover your eyes, nose & mouth, blocking the virus from each entry point. Recent studies show that Face Shields protect you from 96% of droplets when you are 18" away and 92% from 6 feet away.
The CDC recommended cloth face masks to protect others from your droplets which occur when you sneeze, cough, and talk. Unfortunately, face masks do not protect You from other people because your eyes are exposed and small particles can also travel through a surgical or cloth mask.
The plastic face shield blocks you from unintentionally touching your face with contaminated hands. Face masks do not prevent you from touching your eyes and many are uncomfortable, causing people to adjust them and fiddle with them, potentially with contaminated hands.
See here for other studies regarding face shield effectiveness.