Here are some piuctures of flex boards we have designed.
Flex boards are great for daughterboards. They can be designed with a long tail for connecting to the main board, which removes the need for a cable and connector. Due to the setup costs, it doesn't make sense to design a custom flex cable for low-volume production but for production higher than about 10,000, they come shooting down in price and can be cheaper than using a rigid board and a separate cable/connector.
The boards can be very thin and bendable, which is nice if it is going into a cramped product or an awkward location. They can be folded and guided into position and can even have an adhesive backing for application directly onto the end device.
If you open up any common smartphone, you'll see about 10 custom flex cables connecting all of the peripherals to the motherboard.
