Depending on the customer and the work, I might not — or, to be more precise, I might only know some aspects of the product's quality based on what the customer tells me. Or I might only directly know whether certain aspects of what I'm doing are good. Arguably that's actually always the case, but if you're doing consumer-oriented mass-market stuff, then it's quite possible that "what the producer knows" overwhelms what the "customer" knows. But not all software is something you can dogfood or A/B test, and software developers are _very_ quick to assume that they know better than customers, especially if the customer is in some way locked-in.