Why do I do it? Because pugs are well-known for tolerating costumes. They love to be the center of attention, and Liam quickly learned that clothing on a dog makes humans squeal with delight, and want to pet him. (He does best in costumes without hats, however, as he doesn't seem to like to have his ears covered. I learned that with this costume, which made him a little unhappy when he first tried it on.)
Many people still consider dog costumes a form of abuse. I couldn't disagree with this statement more.
My pug loves attention. He lives and breathes for people to notice him. Wearing a silly outfit helps him do just that.
Additionally, dogs have no inherent sense of pride. They do not look at themselves in the mirror and think, "Boy, do I look silly. I wonder what the guys at the office will say."
Liam knows the costume means extra treats, extra attention and extra affection from random strangers. Seems he can't lose here.