As the person above. I enjoy clothes browsing and shopping, and the app provides a very good experience, plus you normally tend to get app-only discounts. From a retailer perspective, (if the app is good) they get better retention and order value numbers from app customers.
Because I buy a lot of clothes and the ASOS app is well built and its slightly easier to use than the site.
Most people do these days; it's convenient. Businesses push it because it improves retention too.
Installing an app takes two seconds. It’s not a big effort.