Jackets serve several different purposes in the sport of mountain biking. Rain jackets help keep the rider warm and dry in the event of a thunderstorm. Other jackets are designed primarily for warmth, which allows them to be more breathable than a rain jacket.
However, the best mountain biking jacket is both waterproof and breathable. Trying to achieve both of these things allows the rider to stay warm without overheating, while at the same time not getting soaked to the bone when it rains.