Not every pork belly is the same, and the fattier ones are sold as a cheaper brand. If you've ever bought the really cheap bacon, it shrivels up to nothing as it cooks. So the window does let you know what you're getting and let you make a price/value decision about how much fat you're willing to pay for.

I suspect that brand affinity may have something to do with people not using that window regularly. Once you find a bacon you like it's going to be relatively consistent. But without the window you can't really compare two brands.