It is where browsing turns into a decision, and the images do most of the persuading. A shopper scrolls the gallery to answer what the text can't: how long is it, what's the back like, is the fabric thick, how does it look on someone my size.
A complete gallery usually covers a main image, the back, a detail close-up, the product on a model and one in a real setting. Gaps show: a PDP with only a flat lay leaves the shopper guessing about fit. The catalog photography entry covers keeping galleries consistent across a range.