I could say: I had been shown a footage. Or: I was shown a footage. Or: I saw a footage. I still can say about it using past simple, like it happened at some time in the past. Right?
In most contexts, the use of perfect forms is optional, depending on how you want to refer to the temporal relationships.
When you use a past perfect, you are making explicit that you are looking at the event from a later point in time (often from the time of another event, but not always). If you do so without already having established a later time, this can be confusing (but is often used at the start of a story to set the time, and make clear that the events described are longer ago.
So if you say I had been shown some footage without any preamble, this is likely to be confusing. But if there is already a later time established, then it makes sense: I knew the film was being made: I had been shown some footage.
Past perfect is used when there are two events that happened in the past, but one event happened before the other and you want to make that clear. So in your examples there is only one event: the footage was shown. If you change the sentence it becomes clear: When he arrived, I was shown the footage. There are two events, happening at around the same time. When he arrived, I had been shown the footage. This means I saw the footage before he arrived.