Today's Starliner launch kept the boosters attached for an additional 48 seconds after they burned out. Keeping the dead weight and additional drag is pretty obviously suboptimal.
During the live broadcast Tim Dodd (The Everyday Astronaut) already got the answer by someone in his Discord channel: "They hold the boosters to control where they fall". Certainly that makes a lot of sense if there's something they certainly do not want to hit.
Is that true, and if it is what exactly were they trying to avoid hitting?