Metal work of all kinds needs clamps, and c-clamps are ideal.
They pack a lot of holding power, pound for pound, compared to the long clamps we use for woodwork. A small face works well when the material is rigid.
Mock-ups, holding material together for welding, and clamping down for drilling are my frequent uses.
I keep a 4" c-clamp with my brake tools. My Ford needs brake pads every year or so. I use the c-clamp to compress the brake cylinder before I remove the bolts. The whole thing comes off and goes right back over the rotor very easily every time. My son ( who does my pads for me now that my hands don't work ) can do my back pads and rotors in about 20 minutes on the ground.
Lots of uses, whereas my bar clamps are mostly for wood, but I use them in odd ways also.
Chilly
BTW - if you find yourself in a big box store, or any place where you can see the steel bsr joist construction, look up. Almost all the pipe, some conduit, and other stuff hanging off the structure is hung with a type of c-clamp (a top beam clamp or simply beam clamp.) Fire sprinklers use a 3/8" up to 4", 1/2", 5/8", 3/4", and even 7/8" rod for bigger / multiple pipes.