Why is it important for Bruno to hear Shmuels voice

It is important for Bruno to keep hearing Shmuel's voice in order to not believe Gretel's opinion on him having an imaginary friend and that he has been talking to himself all this time.

Explanation:

Bruno wanted to be a soldier as soon as he grew up just like his father but Shmuel was against that thought of Bruno quoting the statement "You don't know what it is like here". The story revolves around Bruno portraying his daily experiences through words to Shmuel. Shmuel talks about how the striped pajamas indicated the prisoners of the facility and that these were the only clothes they were given to wear. Gretel never believes the story of Bruno.