Was it necessary for Gerald to see the photo?

In Act I, the Inspector reminds Birling that Eva Smith was employed in his works at one point, and, when Birling does still not remember, the Inspector shows him a photograph of her that he says he found in her room. As Gerald and Eric also attempt to see the photograph, the Inspector prevents them from looking at it. When the two men ask why they can’t see the photograph, the Inspector remarks that he likes to work this way, with “one person and one line of inquiry at a time.” At this point in the play, the Inspector speaks in short, clipped answers, which can sometimes be very enigmatic.