The moat appears unnecessarily wide, but since there’s closer points of the exhibit to the fence line, I guess it gives the gorillas privacy if they choose it - which is a good feature of the exhibit and from the visitor’s perspective, preferable to an off display area.
@Zoofan15 It was initially designed that way ages back. It's more a small pond for waterfowl, with the animals in the backdrop (the enclosure used to house Zebras and Waterbuck); and there is a second viewing area (with a viewing glass) to the right. I assume the moat and having the gorillas off in the distance is all apart of the 'safari experience'.