The purpose of this image is to get people to join the army and take part in the 'great war' It does this by showing a young girl asking her father 'Daddy, what did you do in the great war?' The poster or image was made in 1915 so we know it was it was a year after the war had started. so its as if the question is asked to guilt you into joining the army.
The father is looking straight at you so its like the question is being directed at you. Its clear the the image is 'for' joining the war as it shows white roses on the curtains and the child playing with his british toy soldiers protecting the queen.
The question 'Daddy, what did you do in the war?' is written in the style and font used in the 'We want you' posters which were also aimed at getting people to join the war. Also in the type it uses the word 'great' which suggests anyone who fought in the war were 'great'.
The potential audience for the image is really for anyone, young men out of jobs, fathers i.e why the image shows a father with his two children.
Its a very powerful advertisement and i think it would help in getting people to join up to the army also the time in which this image was made was before people were forced to join the army so its a very persuasive poster.