a) To reduce costs by controlling the entire means-of-production
b) To ensure quality control on the product and build faith in the brand, which in turn
c) Pulls people into the store, forcing them to buy a Costco membership if they want the product.
For example, Kirkland coconut water is simply white-label Vita Coco, and the made by label indicates it is packaged for them by the same company that produces Vita Coco.
Does costco get some extra benefit if the consumer buys Kirkland from costco ?
Yes. Costco positions their store brand as a major part of their appeal. This attracts more consumers to their stores, which
A. Increases membership, for which there's an annual fee, and
B. Increases traffic, increasing sales of high margin items as well as the low margin store brand.
If they were to give away their reputation to other sellers by letting them sell their QC'd products, they would be lowering the value of their memberships.