Incomplete, displaced supracondylar fractures are common. Some of them are minimally displaced, stable and cosmetically-important malunion is rare. These do not require reduction and can be treated with simple immobilization.
If the entire capitellum is displaced posterior to the Roger's anterior humeral line in the lateral view, then reduction is recommended. Many surgeons prefer to perform the reduction in the operating room and stabilize with intraosseous K-wires.
An alternative is to perform a closed reduction followed by cast/splint immobilization. It is difficult to maintain reduction without intraosseous pinning.
If clinical or radiographic examination suggests a varus malposition, it is easier to prevent malunion at this point, rather than to treat it later with an osteotomy.