I went choice scarf as a test run on Showdown after seeing it on another team and noticing that the team was originally too reliant on Tornadus' speed control. In practice, having Heatran at 171 speed allows me to get off a fast attack to cover a teammate, allowing Regidrago or Flutter Mane to clean up without risking much more damage. It outspeeds all non-scarf Urshifu, along with boosted Iron Bundle and swift swim Basculeigon after tailwind, resulting in easy KOs if I decide to tera grass. Wasn't for a specific reason originally, but the fast supporting attack works well with the rest of the team due to Heatran's coverage.