The normal reason for using this construction is that the writer wishes to reserve his position. He is not saying whether he agrees with the UN conclusion or not. He merely wishes to report factually on what the United Nations has asserted. When we see this, we normally say to ourselves, "Let's look at the photos and judge for ourselves. We normally cannot either because most of us have limited expertise, if at all, with the interpretation of satellite photos. This is a very good question. You are reading and learning your English meticulously.

There should be some punctuation to clarify the meaning of the sentence : 'The United Nations has released satellite photos which, it says, show the Temple of Bel in the ancient Syrian city of Palmyra has been destroyed.' It could have been written without 'it says' as the meaning, that they provide evidence, is implicit: 'The United Nations has released satellite photos which show the Temple of Bel in the ancient Syrian city of Palmyra has been destroyed.'