Hanging from a bar does provide a good stretch. Doing this type of stretch may be enough for certain activities. However, most activities require other types of stretching in order to make the exercise beneficial. Some people gets cramps or lack the stamina needed to do a complete workout or routine.
Therefore, other stretches may be needed. Simple exercises or a short routine can be done with a little stretching like hanging from a bar. Many people like to hang from a bar because it is simple to do. It may be a stretching exercise that people do because the bar is going to be used for other exercises or there is not enough room to do other stretches without bothering someone.