Summary: UX efforts should improve their quality of data and informed decision-making by making sure feedback is coming from the right users.
Let's talk about fake users
Fake users are the wrong people who will provide you biased, tainted or innacurate and imprecise feedback. Precision is everything if you want to be successful with UX. Getting the right users, priceless.
At Experience Dynamics we learned this from doing it the other way (Hint: Your UX Agency does it this way most likely, ask them). Like many, we used to have a Market Research firm recruit for us, now we do it all meticulously in-house.
Market research firms provide recruiting (focus groups are happening all the time). But it's also very common to have user no-shows, luke-warm relevant feedback or someone that feels like a badly hired actor. We have not experienced any of that for many years-- that's when you know you are doing it right.