A visually polished app screen can still be genuinely confusing to actually use. Think of buttons placed where users don't expect them. Or flows that take five taps to do something simple. Or onboarding that loses half its users before they reach the main app. None of that shows up in a static mockup. It only shows up once real people try to tap through it. That's exactly why usability testing matters as much as visual polish.
We design with both in mind from the start. Wireframes map out the actual user flow before a single colour gets chosen. Prototypes get tested with real users where possible. That catches confusing moments before a single line of code gets written. That order matters. Fixing a confusing flow at the design stage costs a conversation. Fixing it after development is finished costs a rebuild. That's a cost most businesses would rather avoid.