A statement bag can transform an outfit. It can take your ensemble from zero to ten with one shoulder toss. It shows style, sophistication, and can literally act as a shield against the world. Coco Chanel invented the inside pocket in her purses for "women's secrets" and the shoulder strap because "I was fed up holding purses in my hands and losing them so I added a strap and carried them over my shoulder."
Not only does your bag enclose the contents of what you need to get through the day, but it also shows you are serious about how you are presenting yourself, that you know your brands, and that you are worth that bag.
Now let's get to the definition of a statement bag. In general, a tiny bag can also make a statement (like a Celine nano bag or a Louis Vuitton pouchette), but for the sake of practicality for work, these bags are going to be on the bigger side. They also will probably be black, grey, or tan. Red is also acceptable, but you probably want to save your lime green bag for the weekend (unless it is a Birkin). What's important is that they show style, but are also durable. Essentially they will get you through life and make you look good while you do it!