A triathlon jersey is purpose-built for the unique demands of swim-bike-run racing. Unlike a road jersey, it's made from quick-drying fabric so you can wear it straight out of the water without changing, and it won't stay soaked and heavy through the bike and run legs.
Tri jerseys also feature a full front zipper for quick ventilation and easy on-off during transitions, plus minimal rear pockets—since you're racing, not carrying snacks and tools. The fit is snug but slightly more forgiving than a pure aero road jersey, allowing comfortable running.
If you're just doing a casual triathlon or duathlon, a road jersey can work, but you'll likely deal with chafing from a wet jersey on the bike and run, and fumbling with pockets during transitions. For regular triathlon racing, the dedicated jersey is worth the investment.