It's about observing the users fumble through your UX when you know their motivation.
Some time ago we did such a test. We called 10 customers to our offices and had them do some flows in the application. They didn't fumble. They pretty much did what they had to do and left positive reviews.
That whole thing got scrapped because consultants convinced our CEO that qualitative data is not good for global scoped startups, and that we should be building based on quantitative data.
Honestly, in less than a year, our customer experience was already taking a dive because all the extra little features we would add and strange UI elements, it became a confusing mess and our tracked NPS (Net Promoter Score) showed that. I've since left the company, but I check on them from time to time and they never really recovered and continue doing A | B in the hopes of hitting that sweet spot. It's just an unrecognizable monster at this point in my opinion.