Hats are

• Easy to design/make
• Don't require much space in an inventory (depends on the game)
• Are easy to theme according to an in-game event or a away-from-keyboard event (such as Halloween and FIFA World cup)
• Can show off how long the player has been playing the game, or how dedicated he is.

Hats...

• ... are easy to design/make
• ... don't require much space in an inventory (depends on the game)
• ... are easy to theme according to an in-game event or a away-from-keyboard event (such as Halloween and FIFA World cup)
• ... can show off how long the player has been playing the game, or how dedicated he is
• ... can let the player to show off without masking/hiding the face of the character; or it can be used to mask the player's face, depending on the hat.