A shell jacket has no built-in insulation, but that's actually its biggest advantage. Instead of the jacket deciding how warm you'll be, you control it by adding or removing layers underneath. On a warm spring day, wear just a base layer. When it's freezing, stack a mid-layer fleece or puffy underneath.
Shells are the go-to choice for backcountry touring because you're working hard on the uphill and need maximum breathability, then layering up for the descent. They're also ideal if you run hot or ride in variable conditions where temperatures swing throughout the day.
The trade-off is that you'll need to invest in good mid-layers to stay warm. But that layering system gives you more versatility than any insulated jacket alone—making shells a favorite for experienced riders who want one jacket that handles everything from storm days to sunny corn laps.