A lot? What makes you say that?
Some people prefer to have breakfast and a lunch at work, some will have one meal and a couple of snacks, some prefer one meal that covers both breakfast and lunch or will only have snacks. Some may have diabetes and need to carry extra drinks and snacks in case of their sugars going too low as well.
In any case, a lunch bag means my food and drink is kept cool, secure and as far away from the gluten infested staff facilities as possible. Without trashing my actual handbag. And without being quizzed upon what I'm eating, whether it's something they would like, whether I make it myself, how many calories are in it, if I really eat like that every day, asking why I don't buy sandwiches, asking why I don't just buy sourdough if I'm Coeliac because their aunt says it's absolutely fine, where is the chocolate and the crisps, don't you get bored eating that, oh, I could never eat sooooo much food, there's easily 80g of rice there, did I know about heavy metals in fish, etc, etc, etc.
I just want to carry my food and drink easily, keep it in my office and eat it in peace. A purpose designed bag makes it easier - particularly for avoiding those who feel the need to comment upon what others are eating.