Dimsum meals are usually social and a lot of (Asian) families bond together over meals.

In theory, you get food faster from the carts than from the traditional ordering from a menu. The food from the cart is ready to eat. (Beware that some of the food may get cold as it has been out for a while.) Yes, there are cases where you don't get what you want from cart service or menu.

I'm not sure about the saving expenses part. Many of the people who serve the food from the cart system are older women (seniors). With cart service, people get their food faster and can lead to a faster transition from outgoing customers to incoming diners. I think it is more of getting the revenue faster.