Why use clamps?

I have seen a number of builds here where frames are clamped together. Having grown up with a father in the fastener industry I’m quite baffled by this. I’m hoping to be educated on something I’m unaware of with using clamps. We always had clamps around as a kid, his first job was making cabinets. I have seen things clamped temporarily. Can some one explain why you would choose clamps over fasteners? Just in case Fasteners = Screws, nails or other mechanical joining device like rivets.

Thank you for your help