Why should watchmen keep a list of all personnel going in and out of the confined space ?

Watchmen check the headcount for helping and ensuring to people coming In and Out from confined space. It help to cross check with roll call report during accident or any emergency in confined space 

To cross check with the roll call report in the scene of accident or emergency in the confined space. If evacuation takes place register with watchman helps in finding out wether there is anyone who is trapped inside the confined space.