It's a way to be noticed. Some people need that and some don't. Through decorations, they are able to announce who they are without speaking a word. In a work enviroment which emphasizes conformity and consistency, personal expression may express resistance or assert individuality. Decorations, like humor on the job, create a more relaxed and hospitable atmosphere or break the monotony of work with heavy machinery.
Workers take even more liberty expressing themselves on their lunch boxes.
Since the worker owns the lunch box, employers rarely object. However, companies often supply tools and workers are more reluctant to put stickers on the big coolers that they now bring to work.