The statements, falsely, showed that royal associates, employees etc were receiving payments from news organisations to pass on stories about Diana. It fed the narrative that the palace was working against her. From her persepective it convinced her that giving an interview was the only way to get her side of the story across.Silly question maybe but why did he need fake bank statements to interview Diana? I'm confused.
It was sneaky yet clever. Journalism at that time was often not in a nice place. The tactics used were not honourable but were frequently forgiven as they 'got the story'. I'm not condoning this, I'm explaining journalism in the 80's and 90's as I saw it growing up. Don't fogret, Bashir was lauded for his scoop at the time, won awards etc. Other papers etc would have been fuming that he beat them to it. The bank statements were partly how he beat them.