Babies love to study facial expressions and features. A mirror helps them become familiar with their own face and to develop self-awareness. One placed at a strategic distance on a play mat can also help with head control and tummy play. Also to focus and track images with their eyes.
Many baby gyms and pram toys include an unbreakable plastic mirror, but you don’t have to buy a special baby mirror. Use plastic links to hang a handheld mirror from the baby gym, or place a full-length mirror on its side and let them study their reflection.