Do they always put casts on broken bones or only in serious cases?

NO: It depends on what type of broken bone it is. Specifically, it depend on which bone is broken. For example, we do not put casts on broken clavicle bones (collar bones), but we do put casts on broken wrist bones (radius bones.) it also depends on whether or not the bone is displaced or shifted out of position. If displaced, it needs to be put back in place (reduced) and may even need surgery.

Not always: Not always, but pretty commonly. Most fractured bones need some type of immobilization to prevent further displacement, but there are some that we don't use a cast such as fractures of the: clavicle, scapula, proximal humerus, and some finger fractures. Good question!