Coil cars are for protecting cold-rolled steel. This is high end flat roll such as car bodies and perhaps appliance shells. Generally this is very thin gauge and very smooth to take nice finishes, hence the protection. This is the "cadillac" product of the steel industry. When I still worked in the industry, no mini-was capable of making these products for a few reasons. One they generally made their steel from scrap rather than ore and it was often contaminated with impurities and alloys that would not allow to be finished properly. the mills could get the varioius properties much more accurately. Second a cold strip mill is expensive, big, and precise to maintain gauge very accurately. Mini-mills did not have such facilities. Finally most modern cold-rolled is heat-treated to create certain characteristics that allow for deep-drawing (forming into fenders and such) and to give it high strength. Then it is often coated on one side to prevent rust when assembly. One side coating is sophisticated technology. Coating lines and heating treating facilities are also big and expensive.
Hot-rolled on the other hand is thicker gauge and rougher. Such things as bathtubs, chassis for vehicles and applications where very smooth, pretty finish is not needed. These generally ship in open gons or even sometimes on flat cars.
Shipping building uses heavier stock which is called plates, and is generally at least 1/2 inch thick. This is often shipped on flats or in gons laid flat on the bottom. These plates are often as long as the gon or flat. Again this is mostly shipped unprotected. The same types of steel are used to fabricate into girders for construction.
