Most people do not complain about a confusing app. They close it. UX design exists to find and remove those points of confusion before they cost users, and the reasons are mostly practical.
First, outcomes. If a sign-up form asks for too much, fewer people sign up. If checkout hides the delivery cost until the last step, more people abandon it. UX work, from research to usability testing, finds these problems and fixes the flow, which directly affects conversion and revenue.
Second, cost. Changing a wireframe or a prototype takes hours. Changing a feature after it has been built, tested and released takes much longer and touches more people. Testing ideas early with real users is one of the cheapest ways a team can avoid wasted engineering time.
Third, support and trust. Clear flows and good error messages mean fewer support tickets. Consistent behaviour, honest feedback and visible system status make people trust a product, which matters even more for payments, health and anything with personal data.
Fourth, accessibility. Designing for keyboard use, screen readers, contrast and reduced motion makes the product usable for more people, and in many countries it is a legal requirement.
Finally, AI. When a product uses AI, UX decides whether people understand what the AI did, can correct it, and know when to trust it. That is why "UX for AI" has become its own area of work.