A winter cycling jersey is specifically built for riding in cold conditions (0–12°C / 32–54°F), and it does more than a regular jersey plus layers can achieve. The thermal fabrics trap body heat, wind-blocking panels on the front and shoulders cut the chill at cycling speeds, and tight cuffs seal out cold air from creeping up your arms.
That said, if you already own a good cycling jacket and a quality base layer, you might be able to skip the winter jersey. But many riders find that a winter jersey alone handles most cold rides without needing a jacket—making it a simpler, more comfortable option. It's especially worth considering if you ride regularly through winter and want less bulk than a full jacket provides.