Sometimes if someone sees something they might like they could bring more money or if there's not much they like they could bring less? no one wants to show up with a hundred bucks in a sweatfest like warped, I hate when bands pack up and leave early because why would I walk around with shirts all day regardless or not if I have a bag if I'm going to get pushed around and engage in a lot of physical activity I'd rather buy it toward the end. Not that the last part had much to do with the first haha but I've had times where I've saved lots thinking there'd be a ton of merch I'd want and ended up buying 1-2 shirts or nothing when I could have bought tickets to other shows or something else.
I think 9 out of 10 bands take card, if that helps you. A lost card can be cancelled immediately and is free to replace. Plus, it easily fits inside an iPhone case so if you're bringing a phone, you're bringing your money.
I guess it doesn't hurt, I just can't believe there's such a "market" for this kind of info.