UGC shows a product in real life, at a cost close to nothing. A customer's photo of a cake at a birthday says more than a studio shot of the same cake.
Sharing it needs permission. A tag or a repost button is not a licence, so ask the author, keep the answer and note how long you may use the picture.
Example
A café reposts a regular's photo of their morning flat white, with the author's permission and their name in the caption.