Two different answers to the same problem
Both categories exist because no single fabric solves weather protection and comfort at once. A waterproof barrier that liquid water cannot cross also slows water vapour leaving; a fabric open enough to breathe freely lets rain in. The hardshell and the softshell are the two ends of that trade-off, built into garments.
The hardshell commits to the barrier: membrane or coating, taped seams, closures that seal. The softshell commits to movement: a stretch woven, usually nylon or polyester with elastane, treated with a water-repellent finish that sheds a shower but not a downpour. Some softshells add a windproof membrane, which moves them towards weather protection at the cost of some breathability — but even those are not seam-taped and are not sold as waterproof.
The conditions that separate them
Sustained rain is the hard boundary. In an hour of real rain a softshell wets through; a hardshell does not. Below that boundary the softshell wins almost everywhere: it breathes better on the climb, moves better, is quieter, and its light repellency handles mist, drizzle and snow that brushes off before it melts.
Wind complicates the picture. A plain softshell without a windproof membrane lets wind through, which matters in cold; a hardshell is completely windproof. But wind alone rarely justifies the hardshell's comfort cost — a windproof softshell or a light windshirt covers it. The hardshell's irreplaceable case is precipitation plus exposure: rain or wet snow, for long enough, in conditions where being wet is dangerous rather than merely unpleasant.
How the two are actually used together
Watch what people who work in the mountains carry, and the pattern repeats: a softshell or fleece worn as the working layer, and a hardshell — often the lightest one that meets the conditions — riding in the pack. The shell comes out when rain starts, when the wind on a ridge turns dangerous, or when the group stops moving and the wind chill bites.
This is why the light, packable end of the hardshell market exists at all. A shell that is carried nine days out of ten is chosen on packed size and weight first, and its occasional-use comfort matters less. The mistake is the reverse purchase: a heavy expedition hardshell bought for a use pattern that is ninety per cent dry-weather walking.