A good usability professional must be able to identify high-priority problems and make appropriate recommendations—and the best evaluators do this quickly and reliably—but a good designer must also be able to design well in the first place. This is one area in which usability testing has real power. It can hone designers' instincts so they can spot potential usability problems and improve the designs without the cost of formal testing on every project.
And interestingly, many of the most compelling usability test insights come not from the elements that are evaluated, but rather those not evaluated. They come from the almost unnoticeable moments when a user frowns at a button label, or obviously rates a task flow as easier than it appeared during completion, or claims to understand a concept while simultaneously misdefining it. The unintended conclusions—the peripheral insights—are often what feed a designer's instincts most. Over time, testing sessions can strengthen a designer's intuition so that she can spot troublesome design details with just a glance. Simply put, usability tests can provide huge insight into the patterns and nuances of human behavior.