1 It increases strength
According to one study, strength can increase up to 10% percent when a person yells or screams. This happens because the fight or flight response gets activated as the body believes it's in danger.
2 It empowers them
When a man screams he feels stronger. This kind of psychological feeling can help him lift the heavy weights he is carrying easier.
3 Distraction from pain
The act of screaming can shift the person's attention from the pain he is feeling, and as a result can help him lift heavier weights easily.
4 Activating the nervous system
According to a study, screaming while lifting can help activate the nervous system, and as a result help the body recruit more muscle fibers that assist in weight lifting. (See How to strengthen your core muscles?)
5 Forced to exhale
Sometimes the grunting or the screaming sound happens when a person finds himself forced to exhale as a result of lifting a heavy weight.
6 To show off
Some men scream and make certain sounds while training to show off their strength and power.
7 To catch attention
Some men scream while lifting to catch the attention of someone they are interested in. (See Why are some people are attention seekers?)
8 To intimidate other men
Some men scream while lifting weights to intimidate others in order to reach one or more of their personal objectives.