Do you need to put an output into an input? Look at the back of most cameras and you will notice a port to get video out. So…if you want to take that output and get it into your broadcast, you need an input or capture card/device to do that. A capture device takes the output from a camera, laptop, decoder, etc., and turns it into an input to be used in your broadcast.
Like plugging in your phone into your car, a capture card or capture device is a way to plug your cameras or other devices into your system to be able to see and manipulate their signal. You can’t just plug your phone into the cigarette lighter, it has to go into the USB port so your car radio can understand it.