Running a function returning void could often be a fire-and-forget scenario, or it may even emit signals later. So having QtConcurrent::run() marked nodiscard for all return-types means users must use Q_UNUSED (or its expanded from) to ignore the warning it produces.

void doThing();
...
QtConcurrent::run(doThing); // currently: warning of unused return value
...
QFuture<void> future = QtConcurrent::run(doThing); // OK, but...
Q_UNUSED(future); // ... it's unused