Using the punching bag can increase a survivor's strength or dexterity, it is somewhat random which one will be increased each time or sometimes both.
Even kids can train on punchbags, so if you leave one person staying in the shelter, they can still gain strength and dexterity to keep pace with members outside or be sufficiently potent against Attackers.
As you can only practice three times at one's full wokefulness, avoid members with Light sleep trait as they take a very long time to train again.
Normally, newly-cruited members take quite a long time to be able to go on expeditions, so you can seize this time gap to train for them.
It is great for relieving stress too.