This collection is useful for filtering sales. It basically watches for the "compare at" price that we use for sales. When an item is not on sale, the "compare at" field is empty; when that happens, it's automatically included in this collection. Once it detects the field is not empty, that item will be ignored.
The 10% discount customers receive when they sign up for the newsletter is set to watch for this collection. If we remove this collection, customers will be able to use their 10% codes alongside sale prices.