Self-professed experts will tell you that requirements gathering is the most critical part of the project, because if you get it wrong, then all the rest of your work goes towards building the wrong thing. This is sooooort of true, in a skewed way, but it's not the complete picture.
The problem with this view is that requirements gathering basically never works. How many times have you seen a focus group gather requirements from customers, then the product team builds the product, and you show it to your customers and they sing: "Joy! This is exactly what we wanted! You understood me perfectly! I'll buy 500 of them immediately!" And the sun shines and the grass greens and birds chirp and end-credit music plays.
That never happens. What really happens is this: the focus group asks a bunch of questions; the customers have no frigging clue what they want, and they say contradictory things and change the subject all the time, and the focus group argues a lot about what the customers really meant. Then the product team says "we can't build this, not on our budget", and a negotiation process happens during which the product mutates in various unpleasant ways. Then, assuming the project doesn't fail, they show a demo to the original customers, who say: "This is utterly lame. Yuck!" Heck, even if you build exactly what the customer asked for, they'll say: "uh, yeah, I asked for that, but now that I see it, I clearly wanted something else."
So the experts give you all sorts of ways to try to get at the Right Thing, the thing a real customer would actually buy, without actually building it. You do mockups and prototypes and all sorts of bluffery that the potential customer can't actually use, and they have to imagine whether they'd like it. It's easy enough to measure usability this way, but virtually impossible to measure quality, since there are all sorts of intangibles that can't be expressed in the prototype. I mean, you can try — we sure tried on the OmniGo products — but in this phase nobody "imagines" that the thing will weigh too much, or be too slow, or will go through batteries like machine-gun rounds, or that its boot time will be 2 minutes, or any number of other things that ultimately affect its sales.