This is is one of the cornestones of user research: users don’t know what they want and can’t predict what they’re going to do.

When you ask them to predict what they would do, you are at risk of getting garbage data. This challenge comes up frequently in creating tests for features that don’t yet exist, say, with prototypes. Asking users, “do you want to use this” isn’t as effective as getting an idea of their user needs and then designing an experience that takes care of it.

To design the best UX, pay attention to what users do, not what they say. Self-reported claims are unreliable, as are user speculations about future behavior. Users do not know what they want.

From the oldie by goody article by Nielsen: First Rule of Usability? Don’t Listen to Users.